As soon as any Facebook app convinced a user to pay, all other facebook apps would have a seamless checkout.
Getting user financials isn't easy but we've seen the rise of Venmo, Robinhood, and Neobanks since then so it's doable.
Desktop penetration is lower outside of the US. Aren’t Internet cafes still a thing in much of the world because people don’t have computers at home?
Not on iOS where that sort of thing is prohibited. Sure they could put a skin over safari, but apple would have banned the entire FB app as soon as they found a developer's app they didn't like.
Yes.