Dennis,
I hope I can shed some further light on your questions as I am currently doing exactly what you have suggested.
The 6½ months for the ATPL groundschool is probably just a ballpark figure they have come up with. I started my studying in March and plan to have them finished by the October sitting, so it will have taken me 7 months. I know some paople on the APP have taken longer though by doing a residential course back in the UK. I am staying in Florida while studying and do so for about 6-8 hours a day Mon-Fri, then try and get some flying done on the weekend. This way I am staying current with my flying and when I have finished my groundschool, most of my hour building should be complete also.
With regards to resits, I unfortunately missed Met by a couple of percent in June but have started ploughing into module 2 already. I had thought about resitting it in Orlando but was not happy about paying $400 for the privilege so will take it again with Module 2 in October. My one piece of advice here is to work hard at them and have a bit of confidence in yourself. If you thinking about failing exams before you have even started this is not good!
With regards to visas. Initially you will apply for an M1 visa, then after the year is up you convert to a J1. This will then give you a total of 3 years in the US.
For trips back to the UK. You will need to make 2 round trips for the groundschool with Bristol, as the classroom element is compulsory. You will then need to make another trip to sit the flight test for your IR in Shoreham, but I imagine you could do this on your return after instructing.
Well I hope this answers some of your questions.
No sponsor, were you on the Mod 1 brush-up in May?