I am guessing that Celebrian was based on Austen's Emma who was a prodigious matchmaker. Her character in the story was meant to be seen as high-handed. She had very few redeemable features and her meddling in the affairs of those around her nearly led to disaster for all concerned. It was only through those mistakes that she finally realised just how much harm she both did and could have done and also could have lost the love of her life who she alienated earlier on by her ill thought out actions. In that was her ultimate redemption. She learned her lesson and was rewarded by finding her own heart's desire. She had been so busy sorting everyone else out and doing it very badly, that she failed to recognise that someone's heart was actually engaged where she was concerned.
Celebrian doesn't have that redemption. She is intefering shamelessly and using her desire for her friends' happiness as a justification. She further complicated an already complicated love triangle by meddling in something that shouldn't have been her concern and by doing so plumbed the depths of her friends' own inner nightmares, fears and self esteem, something that should not be done lightly.
In essence she did exactly the same things that Emma did, but did not have any kind of redemption at the end of the tale. She learned nothing other than the maxim 'it all turned out right at the end', so there was no lesson learned and she would probably do it all again with another set of friends, but next time it might not be all's well that ends well and that's the risk she takes. There is no prize for her ultimately realising the error of her ways. She already has the love of her life, so what has she learned from it all? Absolutely nothing and she would probably do it all again in a heartbeat.
Maerdess is arrogant and highly self-opinionated and her assumptions regarding Glorfindel based on her arrogant belief that he just HAS to love her because she is special and high born lead to her downfall and humiliation. She did it to herself of course as your description of her part in the story clearly shows. However, even through her attempts to redeem her previous actions she still comes across as arrogant. She tries to mute her behaviour and steers clear of Glorfindel, but she does it for the wrong reasons. She does it because she is humiliated and embarrassed because people were making fun of her, but underneath she hasn't changed at all. And Elves have long memories of course.
Even at the end when she is planning her long awaited wedding to the man of her dreams she comes across as smug and self-satisfied, Personally I would have stuffed her wedding dress material down her throat, not offered to make it for her.
I am honestly not sure WHAT you would do to redeem her and make her seem less unpleasant, but making her end up with the glorious Glorfindel isn't it. They are so wrong for each other it's unreal. Two selfish, arrogant individuals together in a marriage would be a recipe for disaster. Even in Austen's Pride and Prejudice, she did not put two characters of the same personality together in a union. Darcy was arrogant and full of pride but Elizabeth was neither. Her problem was that she just couldn't see past her own prejudices.
She eventually redeemed him by example through her selfless actions regarding her family, letting him see the benefits and qualities that she had in abundance. He redeemed her by finally selflessly assisting her family, not to get into her good graces, but because he was still hurt by her rightful suggestion that he was an arrogant prig who thought of no one but himself. He wanted to show her that he had changed.
With your budding romances I would have perhaps gone in quite another direction and had Maerdess end up with the gentle, courteous and kind-hearted Erestor because he at least could have taught her something.
How to be a bit more humble perhaps?
By the same token I would not have put the more gentle childlike Losgael with someone like Glorfindel, so you had the right of it there when she failed to get his attentions. Their union would have been a disaster and she is in no emotional position to take on a relationship with anyone, let alone someone with Glorfindel's monumental ego. She would have floundered and drowned in that relationship. She should have remained unmarried, at least until she came to terms with her childhood traumas.
As for Glorfindel, well I don't think Tolkien intended him to be seen as a large, overgrown child with an inapropriate sense of humour and an ego the size of a small planet because of the warrior/Balrog Slayer reputation, bumbling through people's lives and acting like an idiot to the point where he did actual emotional damage to other people. However that is the way that Fanfiction writers apparently see him now. Even my story has him as not so much arrogant, but certainly with a very inappropriate sense of humour which I attempt to offset by his keenness of perception, wisdom and ability to sense when others need emotional support.
This is Tolkien's description of him in Lord of the Rings:
"Glorfindel was tall and straight; his hair was of shining gold, his face fair and young and fearless and full of joy; his eyes were bright and keen, and his voice like music; on his brow sat wisdom, and in his hand was strength."
He certainly didn't appear to be demonstrating much of that wisdom in his dealings with Maerdess and Losgael, nor even with Celebrian. Perhaps if your Glorfindel had called upon his innate wisdom he might have seen what damage he was doing by his actions. My only suggestion with regard to him is to make him less pompous and arrogant and give him more self-effacement.
You can still give him a boyish sense of humour, but temper it with wisdom and an ability to perceive the feelings of other people. The Glorfindel in your story doesn't feel strong to me. He feels sulky and weak-willed and even when he finally decides that Maerdess is what he wants, he doesn't use his wisdom in the courtship. Instead he goes in like a bull at a gate and quite rightfully gets a slap upside the head for his pains.
So, in conclusion:
You can still make Celebrian match-make, but let her do it with humour. Make her at least a little self-deprecating and let her also understand at the end that far from bringing her friends together in love, her meddling could have hurt someone. Not all things turn out well at the end.
I would have let Losgael just be herself. She went through a series of self flagellating emotions and at the end would have seemed a stronger character if she had finally put her foot down and said 'enough is enough' rather than let herself to be coaxed into a convenient relationship with Erestor who certainly doesn't deserve the potential timebomb that he has been landed with.
Maerdess should have self-redeemed herself to the point where she FULLY realises Glorfindel is totally wrong for her. Her interactions with the Imadris Rumour Control Mill should have taught her a lot more about herself and how others see her and just going quiet wouldn't be enough to alter her mindset.
Losgael's gentle good behaviour, quiet sacrifice of her own dreams for the geater good and gentle friendship should also have taught Maerdess something, but instead she ends up getting the guy and acting smug. The last thing she needs is a marriage with the smug, self-centred, opinionated Balrog Slayer, because they will end up striking sparks and grating each other. He can teach her nothing. Erestor however could have taught her how to be humble and many good relationships are based on opposites attracting each other. He could have coaxed out the nice part of her that had started to appear, whereas all I see in a marriage with Glorfindel is them both constantly trying to get the upper hand over each other.
It's very tempting to make a story have a conveniently happy ending, but life generally is not like that and no matter how hard I try, I cannot see the three couples, Elrond and Celebrian, Erestor and Losgael, Glorfindel and Maerdess sitting down to dinner together all lovey dovey and cutsie pie. One wrong word out of Glorfindel would have Maerdess upending the soup tureen over his head and rushing off in high dudgeon, screeching "You never really loved me!" as she hurtles out of the door.
Celebrian would then take it upon herself to try and bring them back together. This would lead Glorfindel to grab Elrond aside and tell him to stop his medddling wife from intefering in his marriage, whereupon Elrond would bristle and try to defend his wife despite the fact that secretly he agrees that she meddles too much. He would then lecture Celebrian on the problems attached to meddling and she would then get annoyed with him and not speak to him for days on end.
In the midst of this Erestor and Losgael may draw comfort from the the fact that they at least rub along well each other, until Losgael comments to him that she feels that Glorfindel didn't REALLY deserve being covered in soup or locked out of his bedroom by his wife for a week. However this may then fan the flames of Erestor's secret fear that he was always Losgael's second choice and that her championing Glorfindel means that she still carries a torch for him.
And all that, of course, is another story entirely! Oh whant a tangled web we weave when first we practice to deceive!
;)