I've never had shorts rip from sitting. It is possible that you got just a defective pair.
Cold weather gear is very individual, depending on your tolerance for cold and your sweat rate! Most people go for layers on the top. As far as bottoms, I use regular cycling shorts under thermal running tights, of which I have 2 different weights. I also have a pair of knickers as well as knee warmers. Wool socks of varying thickness, toe covers, booties, or winter cycling shoes. I have everything from arm warmers to wear with short sleeved jerseys, regular long sleeved jerseys, thermal LS jerseys of varying weights, a wind jacket, and a very thin, but windproof and warm Gore cycling jacket for winter, with pit zips. Oh, and base layers, both LS and sleeveless, wool and regular wicking to wear under any jersey for added core warmth. Skull caps of varying thickness and a balaclava, which I hardly ever use.