I was going to suggest trying a different web browser, but were you logging on to Facebook through a browser or the app? It would seem odd if you have a similar problem with the FB app and a web browser, because those should be entirely independent (the keyboard would be common to both, which is why I was wondering whether passwords were being entered correctly).
I've used EU market phones in the UK without problems, so it shouldn't be that.
I've not played that deeply with the S8, but normally when you are asked which app to use I'd expect to be asked whether to use it 'just once' or each time. If it didn't ask whether you wanted to make your choice permanent that's odd.
Would a factory reset fit it? I don't know because I don't understand what's going on. Working out one oddity is one thing, a couple if a connection can be found, but there seem to be a number of unrelated things going on here. It's possible, but there's also the risk of going through the hassle of setting everything up again for no gain. If it was new then it should have been clean when you got it, so anything a reset could clear would be stuff that's been added while you've been using it.
Whenever I've had to return stuff to amazon I've just contacted amazon. You bought it from amazon, so distance selling regulations mean you have the right to return goods within some period (I think it's 2 weeks) no questions asked. They generally have a good reputation for returns.
