OK, I'm going to take each of these in turn:
But it’s really not difficult to say ‘thank you’.
For anyone.
Actually, it is. If you don't have the social script, then you don't know what prompts a thank you and what doesn't prompt a thank you. Think of it like this: Most people, when they're walking along, don't have to think about picking up their feet, in the right order, and putting them down again, in the right order and in a particular way. It's just an unthinking thing that we do. We half-knowingly, almost subconsciously, avoid hazards. My son is epileptic. That bit of his brain isn't there. It's not automatic for him. Therefore, every step that he takes is intentional. Now take a load of the social things you do, and imagine that every single one of them has to be intentional. It actually is quite difficult. Some understanding around this would be appreciated by all those autistics who find it difficult.
You 'stopped putting her through the pain of surprises' and you think this will help her grow and be comfortable in the world she will live in for all of her life?
Yep. She's now in her 30s and earns an extremely high wage working in IT. She's pretty darn comfy. Engaged to be married. Your point is, therefore, lost on me.
You're right, it is exhausting, but that doesn't mean you don't have to do it. I'm autistic too and I am getting increasingly sick of people saying "but I can't possibly be expected to do that, I have autism."
Thing about autism is that we don't all have identikit personalities. The people annoying you, who also annoy me, are basically 'refuseniks'. But there's a difference between excuses and reasons. An excuse is something that's using some weird flimsy evidence, e.g. 'I don't have to do that because I'm autistic' or 'You should do that because it's what nice people do.' Both are just junk. There's no logic in either. A reason, on the other hand, is able to use evidence and explanation to prove its point, e.g. 'I'm autistic. I find surprises really hard to deal with. This is probably because I have slow processing speeds and this means that I panic when I'm put on the spot and so shut down or seem to say things without thinking. Therefore, it can be helpful for me to have thinking time as this effectively stops me blurting stuff out that I don't need to say and you don't need to hear. Thank you. Anything that I can do to support you, as well, please just let me know.' This invites people to me, in advance, and reduces surprises. Hope that helps.