Re: Two questions on starting a sequel
Just to be awkward, if not downright contradictory, I'd go for a combination of points 2 and 3 -- jump in the deep end but with a bit of explanation of what has happened woven in unobtrusively into the narrative as the book progresses, though not dumping it all in chapter 1. Even if people have read book 1, they might appreciate some kind of reminder of the salient points, since they're unlikely to have refreshed their memory immediately beforehand.
As for POV, I think readers are sophisticated enough to work out who is important -- just because the first POV of the book proper is from a secondary character isn't going to faze them unduly. Those who have read book one will know who is the main character; those who haven't won't know whose POV started book 1 in any event.
Re the writing a second book before the first is sold, that advice rather depends on how quickly you can write the second and subsequent books. Publishers are, I understand, likely to want the books to come out in successive years. If you can write and completely edit a sequel to publishable standard inside 9 months, that isn't likely to cause a problem. If it takes nearer to 15 months, then my advice would be to ensure you have at least one follow up substantially written, so you are ready for the deadlines.