Help with tires/wheels/alignment

[Hoser]

I've got a 3 week old (to us) 2001 x5 4.4i with 20 inch wheels 275-40-20 Frt & 315-35-20 Rear. Just had a flat on rear after 3000 km vacation pulling 3500lb boat. The inside of the tire was worn right down to the steel cords. This wear was only about 2" on the inside of the tire (driver's side) the other side rear also shows a lot of wear with cupping- but again only 2" or so on the very inside edge. When I had these tires on, the camber was very pronounced. I have since changed over to winter Nokian Hakapalitta 18". They ride so much better, I'm thinking I should change out the 20's for 18" for next summer. I do want to address the rear alignment and will be getting a four wheel alignment. What is the preferred setting for camber, toe in, caster on rear & front? Any suggestions if I keep the 20's on how to relieve the harsh ride? Does lowering tire pressure help (was at 32 psi)? Thanks to all who share their wisdom.