I don't know why you're being called ridiculous, unhinged, told to get a grip etc, for some people it's natural to worry and overthink this stuff, especially with a first baby.
I have 3 kids, and with my first I was definitely overthinking everything. It's much easier parenting the next ones once you gain some perspective. The first time is more stressful, so give yourself some grace. Also although we have 3 kids, we still do no screen time for baby, and only a weekend show/movie for our 2 older kids.
You're right that the guidelines that came out say 0 screen time at that age, so it's not crazy to be mindful of it, but also you could try to put it in perspective that it's only 2 hours one time, and the guidelines are probably because of babies being plonked in front of TV loads daily and missing important social interactions, floor play etc, not because of a one off.
Personally I wouldn't say anything since it isn't a permanent arrangement (although I also wouldn't be thrilled about it, I'd keep it to myself). It's a lovely thing she's offered! I think no matter how you mention it, it would sound ungrateful/rude.
For a 7 month old 2 hours is nearly a whole wake window, could baby be having a nap while you're out? If it's not during his nap time, I would just pack a couple of small toys/books and leave them there for them, so they could play even if TV was on in the background.
It's great you get some time with your husband, hope you have a lovely time!