MrUD you asked some specifics I feel I'd like to respond to. These are in no way getting at either of you. But you've posted an AIBU, which at heart I think are the simple question, what does everyone else do. Here's my take on it:
'But AIBU to be upset when she tells me how things are going to go, and then it turns out completely differently, not just once, but every single time she's gone out?'
Lucky her for having some spontaneous time. What a very adult joy.
'AIBU to ask for some reassurances?'
Sorry that's a bit vague - that she's ever coming back? She is, you know. That you are good enough for DS? You are, should go without saying, you could probably do with building your own parenting confidence. That she should come back and be a reassuring human dummy for a very lucky little boy? Yes, that's unreasonable. Try it this way: you would have to cope with (deity etc forbid) major illness/accident interrupting a nice smooth first year. Wouldn't you rather feel more prepared? Building your own abilities is the best assurance around.
'AIBU to be concerned for our baby, and be wondering whether I should put him down for a few minutes to boil a kettle to warm a bottle, when he's crying, but decide to keep holding and singing to him because I think UD will be back in a few minutes - but then she isn't?'
Wouldn't be concerned, no. Missing a reassurance nighttime feed is not sending him to developmental hell. Yes, its always a shame when you can't use the easiest and most familiar 'dealing' method in any situation. But you are being unreasonable at 10 mths to be fretting such a small detail, figure it out! Just offer the bottle! Get in there every night until she goes and get ds addicted to your particular song/rock whatever it is.
'AIBU to have desperately wished that last time she went out, I would be able to trust her to keep to her word, but feel really crushed when she didn't?'
Erm, yes. 'Desperate'/'crushed'. Resolve the stuff about lack of confidence in your skills, and this should dial down to a more reasonable irritated/miffed level!
Sounds like UD and you are the victim of your previous parenting success, but it's time to move onwards and upwards?