If it's made for exterior use and has UV protection, yes, you can use it.
sure can but use Marine grade that is made for; more expensive but you won't have to keep spending $ to do it again when porch/deck paint flakes off; after stain you still need to seal it with something to prevent rot.
Here is a website that can give you the answer along with reviews on some products you can use... https://www.deckstainhelp.com/apply-polyurethane-to-a-deck/
It may make it slick. Ask a paint store what is best for your area.