Asklemmy

Sounds all good, yes. That's generally the way I do it, except I prefer to also grab a paper boarding pass at one of those automated kiosks at the airport. Like the others said: The QR code should be enough on its own, though.

If you have luggage to check in, you show them your boarding pass, so that it can be registered to your ticket.

Depending on where you are, you might need a valid ID with you. USA is a PITA that way. Here in Europe you can usually fly without ID, as long as it's domestic.

Tip: If your ticket includes it, you can probably select seat in advance so you're not stuck in a middle seat at the back. Preferences vary, but I prefer a window seat as far in front as possible.

Every time I've checked in online the qr code is the boarding pass so there's no need to go to the kiosk, they check it on your phone. But as the others said it may vary depending on airline or country. And you probably don't need the app at all unless the website says otherwise.

Difficult to say, and depends on other factors, like your airline, and if you have connections. However, assuming you have proper ID, if you run into trouble I have little doubt the kiosk for your airline can't help you sort it out.

I do still use my airline's app (Android) for domestic flights, because it supplies me with other up to date info, that has saved my butt a few times. I just make sure it has limited permissions.

Depends entirely on your airline