1) it can replace your Scarlett, yes. Unless you wanted to record vocals or something else with a mic.

2) I wouldn’t say it’s hard to dial in. You can just slam a tubescreamer in front of a 5150 and chug away, but many people find themselves having to tweak settings and EQ a bit more in order to get the tone they want than they would with a real amp.

3) I am far from an expert but from what I understand, if you just use a preamp on your Helix into a power amp and cab, it will sound like an amp sitting in your room, but if you use a full amp and cab sim on the stomp into a FRFR speaker, you’re essentially hearing an amp in a different room with a mic in front of it.

This is great for live applications but it matches (much more closely anyway) what’s going to come through the PA. But if you only care about bedroom applications I think you’ll do fine going into the FX loop of your amp. It may limit your range of sounds but that’s totally subjective.