Paul
Sounds like you are refering to the fillet's that hold door frame to the outer skin? If so:
1) Aluminum
2) Don't have micrometer thinner than skin I beleive based on memory
3) Physical roll of outer skin holds it all together
4) Looks like rivets are about 2.5" in spacing? Probably all dictated by the guy doing the mounting?
Door rivets.....
Best of luck