1) Yes, if you pay for a comfort seat. You don't pay the full whack of taxes so seat2 is cheaper than seat1. You have to call up to book it, it can't be done online.

2) You get one boarding pass, one load of Avios, the system is supposed to block off the seat, but there is always a risk of a gate agent or cabin crew member trying to be helpful and inadvertently taking the seat. On FLY it is easier to spot than on PRS, or so I've been told.

The one thing you should not do is book 2 seats online. If they are not specifically marked as a comfort seat then the unfilled seat is regarded as fair game for standby passengers. I had someone near me on a Newcastle - London flight try that a few days ago, and he only got away with it due to it being a low-loaded flight. He did check in online so it caused a very short delay but he nearly wasted his money not achieving what he wanted to do.