I think I have read most of the thread, but had to go out quickly so may have missed some, apologies if so.
Just to say that I have not had to support a relative through their death, but am a speech and language therapist working with adults with swallowing problems, quite often at end of life. I don't regard myself as an expert, I'm learning all the time.
What worries me is that Bill doesn't say that he is raising this with Mary's team. I hope he is, and is just getting extra support from you - I'm sure it's incredibly helpful. But he should feel able to say that he's worried and ask his questions of them, and get an answer that makes sense for him.
I also think that Mary gets choices here - she may well aspirate (liquid going the wrong way) on water, but she has a choice as to whether the aspiration or the dryness is more uncomfortable - she should get the chance to say what she wants. If she's not able to do that, they should be willing to try both and see how she reacts. Drinking/sips at risk of aspiration is her right if she wants, as long as she understands the risks.
It's very unlikely that Mary will want to sit up and have huge cupfuls - the doctors/nurses may be able to give Bill an idea of how many sips per hour/day might be OK without running into worse problems down the line.
I, and other members of the team, often try ice chips with patients in this situation. Many wards will have machines that make tiny pieces of ice, if not, then ordinary ice in a bag bashed up would be fine. An ice chip is a tiny amount of fluid but the oral pleasure and relief they give is out of all proportion to the fluid amount. Agree also about ice lollies. Sittng up is a really good idea to reduce aspiration if she still can, but obviously not at a cost of increased pain.
If you can encourage Bill to say as often as he needs to that he's worried that Mary is too dry because of her consistent signing for sips, that would be a good thing. It may well be that she can have fluids under the skin or other things. At least if he asks and gets an answer that makes sense, he should feel less traumatised than others on this thread have been 