You can wear a blouse under an OP but make sure the necklines go together. A blouse with a Peter Pan collar under an OP with a Peter Pan collar would look weird and bulky, but if you were to wear a blouse with a collar under an OP that just had a square or sweetheart neckline that can look really cute. Or if you wore an OP with a collar with a blouse that didn’t have a collar or if both pieces didn’t have collars.
It doesn’t look like it will fit over a petti at all.
I think the Baby coat would probably work fine. It is more Sweet but it wouldn’t look terribly out of place in a Gothic coord. The other jacket may be possible too but it’s hard to say without actually seeing it.
The thing I would worry about the most is the ruffles because I can’t tell how durable they would be. I think overall they would probably be fine but I would definitely suggest spraying them with waterproofing spray before wearing them in wet weather and maybe getting them half a size bigger than you need to give you room for thick socks on those particularly cold days.
It’s definitely possible and I’ve seen it done quite a few times but it depends on the OP. If your OP has a collar already it’s usually not going to look good to shove a blouse under it. But if it has short sleeves and no collar (a square neckline, sweetheart neckline, or something like that) it can work.
I really don’t know much about skiing but I do know that doing it in a dress is probably not a good idea. So normal ski gear would be your best option, but choose it in colors that you could see in a Classic Lolita wardrobe. Muted colors like dusty pink, mint, or blue, ivory, brown, wine red. Maybe something with a fur trim for an extra little luxurious touch. Then you can work more Lolita themes into things like your hat or earmuffs. Something else you could do if you’re at all artistic is maybe paint a design on the frames of your goggles. Maybe a pretty floral design or something based on your favorite print.