It’s not unusual to have to go around due to another aircraft still on the runway, but generally that’s because the previous aircraft is tardy about vacating. This is altogether more serious.
Not a good idea to assign blame as generally everyone involved on the ground can claim a bit of responsibility – but Barcelona ATC is particularly chaotic and the taxi layout can be confusing.
Also – how easy is it to pull out of (a flattening) curve with enough thrust without other issues (stall)?
Not difficult at all. Even from idle thrust the engines will develop full rated power within about six seconds and a missed approach can be initiated at any point before the thrust reversers are deployed.
Obviously, if you’re at idle thrust and you start the go-around at 10 feet you’ll probably touch the ground in the process, but that’s the worst of it.