Re VAT treatment of N-reg you need to speak to some people who are actually in that line of work: N-reg registration agents can advise on the correct paperwork, and also a corporate accounting type who knows this well, not a street corner personal taxation type
Re syndicates, an alternative to the traditional syndicate is straight renting. One or two people buy the plane and rent it to the others. Unexpected costs are well controlled during the warranty period, and after that you can do other things. It has the benefit that the owner(s) (and somebody has to be in charge; there's a lot of keep track of when owning a plane which frankly most flyers don't want to get into) have full control and getting rid of difficult people is easy. And renting is a commercial activity so you can claim back the VAT. Being a VAT registered company (best do this as a ltd. co) you have to charge VAT, but that's no worse than in a syndicate.