Being withdrawn from humans, not being receptive to taming, and general inability to entertain himself (i.e not a lot of playing with toys by himself) would all be signs of depression/wanting a friend.
As for Tilda, Mallorn's really strange with her. She likes talking to her, and screaming at her as well, through the cage bars. Also, she likes to try and hijack her stuff through the cage bars. We've tried in the past to let them out together (under supervision) because they seemed to get on well when separated by cage bars, but Tilda was just much too passive and let Mallorn walk all over her (literally!).
Mallorn, bothered by the lack of response from Tilda, would try and step on her and chase her around. At some points, it seemed like she was trying to "make friends", but Tilda was having none of it and Mallorn just got frustrated. Besides, she hated when I spoke to Tilda when she was out with her and would land on my shoulder to scold me afterwards. Needless to say, they aren't allowed out at the same time anymore, Mallorn's too much of a handful. She still likes visiting her through the cage bars, though, and Tilda is just fine with that
I don't think I would ever recommend housing a diamond dove and a budgie in the same cage. Doves are so passive; they are almost the polar opposite of budgies, and compared with the rambunctious budgies I don't reckon it would be good at all.
Tilda is very zen, she values her peace and quiet, and although she has bonded in an amazing way with my sister, she still wouldn't appreciate having any more budgies near her
Oh, and about the quaker and the budgie: that is never, never, never, never recommended, even if the birds are bonded! A Quaker is huge, and like all roommates, when the two birds ever argue, even a light peck from a Quaker could be fatal to a budgie.
Ana (aluz) has a budgie and a lovebird who are bonded, but despite this, they are not allowed to share a cage because of this.
Does that help?