1) No answer to that, 10 people will tell you 10 different things, the same as with any other products it's just something you arrive at by your own trial and error
2) Yes, except I'd not use a carpet brush on a sofa, maybe tampico
I use a grandi groomer on carpets to work it in (and the handi groomer on stairs)
3) Yes, obviously because you're applying more moisture. Important point - do NOT turbo dry protector, because it needs the absorb into the fibre. Dry it off too quick and you'll stop it being absorbed fully. I always turbo dry straight after cleaning so the carpets are just a tiny bit damp before application. This stops long drying times.
4) No, they won't take a blind bit of notice of it anyway
I do have own label spotters which I leave behind with every customer. I think simply stopping them reaching for the vanish is much better and realistically more achievable than getting them to follow any set instructions. The instructions are on the spotter bottle anyway.