It's not about returning the trust or not returning it. Hikari likely never had any difficult decisions to make in their relationship (though we are yet to see), so there was no need to omit or lie about anything. I agree that Hikari trusted Yami, it's just that the weight behind truths they can share is vastly different.
As for Yuu, it's less about trust and more about the commitment. "Beach" chapters show the difference in commitment to the relationship they had, especially that episode with Yami proposing to live together and Yuu just not giving her any reaction at all. Again, it's not like he is to blame or that Yami did the "correct" thing, it's just about perspectives and why Yami didn't want to drag him into her mess.
I have an opposite experience. Telling the truth sometimes hurt relationships much more than not sharing it. And it was not even anything malicious or even anything "objectively" unpleasant, it's just that there are truths people don't like, and one of the keys to relationships is finding the golden mean between brutal honesty and secretiveness.
The bigger the thing the MORE IMPORTANT it is to say. If, for example, my buddy was dating a girl I used to date I 100% would tell him we used to date, because if lied about it and hid it what are the options there? Avoid being with that friend again so that it never comes up? Walking on eggshells during any conversation? Stacking lie upon lie whenever they ask me for advice? Especially if something major happened like say they cheated during our relationship or I did something awful to them that could cause lasting effects on my friends attempt to be with them.
I’m not saying you can never tell a lie ever or you’re a bad person, but this is like THE THING you need to have that difficult conversation about. This isn’t “mom I cheated on my math test” or “sorry those jeans really don’t flatter your figure” this is “hey I have serious history with the person you love, this is going to effect all three of us in a major way so I’m going to give you a heads up rather than let you stumble in like a lamb to the slaughter.”
I honestly can not think how you could be friends with someone and justify lying about something like that? What was Yami’s plan here? You asked me what Hikari should have said so I’ll ask you: “What what’s Yami‘s plan to both never tell Hikari the truth and simultaneously not hurt her?” Because Yami suddenly pulling away from their friendship would hurt her, Yami secretly confronting Yuu would lead to issues between him and Hikari that Hikari would have no hope of dealing with uninformed, hell even if somehow Yami and Yuu’s encounter didn’t explode the way it did and they somehow managed to bein the same room together with Hikari the tension between them would be massive and Hikari would be stuck in the lurch wondering and anxious.
Theres no outcome to Yami keeping it a secret that ends well beyond the short term aside from solely for Yami.
And again I’m not saying she has to go into great detail or even say “I still love him” if she really wants Hikari and Yuu to be together. A simple “Hey Hikari, Yuu and I used to date, and it didn’t end well, I ended up really hurting him and I don’t feel comfortable seeing him again right now.” Would yes have made things a little uncomfortable but it would also allow Hikari some agency to deal with the situation in a way that can make both her friends comfortable and importantly not hurt each other.
If my buddy told me “Hey that girl you’re seeing, we used to date.” I would be confused but I’d be able to adapt and yes I’d probably have questions but my friendship would stay intact and I’d be thankful in the long term. If I found out 10 months down the road that they used to date and that he knew and never told me, I’d probably be a nervous wreck because obviously something was happening that he wanted to hide. That shows an intent to deceive which implies Malice even if it’s not there.
It’s always an easy opening for misunderstandings. Things can spin into gossip and gossip becomes a whole fucking lot more believable when you learn that the person HAS been lying to you.
Yami is playing Hikari for a fool and that’s not something you do to a friend. You mention the gold mean between honesty and secretiveness, but Yami has never once approached honesty.