FWIW here's my take:
Re pronounciation: It seems to be the norm in this area is to say "breeks"; I think the more formal pronounciation is "britches".
Re jeans on English saddles: The only things I could see that would possibly damage the leather would be the designer (eg rhinestone, etc) pockets, as already mentioned. A distant second possibility might the inside seam of the jeans - some styles have the double seam in the inside and this, because it's thicker, may rub (I haven't bought breeches for awhile so I don't know if fashion has changed but mine are styled such that the inside seam is offset and doesn't rest against the saddle).
Re difference between jodphurs and breeches: It's mostly in the lower leg; jodphurs are ankle length, frequently with a cuff and sometimes (especially for the young rider) fitted with a strap that goes under the outside of boot and around the bottom to keep them in place, and are worn with ankle high paddock boots; the lower legs of breeches are shorter and minimally designed so that they fit inside tall riding boots.
P.S. I think as long as your clothing is safe and comfortable, you wear what you want.
Last edited by Chevaux; 06-27-2013 at 10:53 PM.