1) We didn't do doublers until baby was several months old, because she pooped often and wasn't a heavy soaker until she was bigger. I always changed poopy diapers asap (to avoid diaper rash), and typically the nighttime changes were poopy ones (because she'd poop with each feeding and she nursed every couple hours at night).
2) I agree that you should definitely register, so that way when they say they want to get you something, you can tell them where you're registered. One thing we did was comment on how we weren't comfortable with Made in China products, because of the recalls and ongoing safety issues. But I always tried to phrase it so it was an appalling irritation for us (as it should be for everyone, etc.), and less like it was an arrogant "our child is too good for plastic crap" type thing.
And/or you can point out how you've found these great natural toy makers (get some catalogs to have on hand or bookmark some sites), or if there's a toy store where you live that sells natural type toys, etc. When in doubt, blame things on your ped or on books you've read about how certain plastics are dangerous (and outlawed in most countries but ours), etc.
I also made a point to vilify my husband when it came to batteries.
I'd tell my ILs pointblank that he hates changing batteries, so no plastic toys please.
Anything we received that we weren't comfortable with, we returned to the store or gave away. We didn't tell the sender, and we always thanked them graciously, etc.
3) Gah we stayed for two nights. It suuuucked. It was because my water had been broken for nearly 22 hours when I gave birth and despite the prophylactic a/bx they gave me (that I should've refused because it led to two months of thrush from hell), they insisted we both remain and continued to do blood pricks on us both to test for infection. This time I'm conveniently forgetting what time my water breaks, if it does.
It's not that they keep the moms for 24 hours typically (at least where I live) -- it's the baby. Your midwife/OB may be fine with discharging you within 12 hours or sooner even. The ped may as well, but I'd ask ahead of time because in most cases in the US it's 24 hours. Our ped's super laid back and he likes a minimum of 24 hours for baby in the birth center (which is connected to a hospital but a separate entity).
4) No nursing shirts here. I don't even like the button up front type shirts anymore honestly. I am a shirt-lifter. It's easier and more modest. I can't imagine buying a designated nursing shirt or gown or whatever. But I like things very simple and that adds too much fumbling for me. Definitely in the middle of the night you want the simplest arrangement possible so you can side-lie nurse and keep on snoozing.
YMMV. But I'm not a fan of complicated shirts.