2) How to best splice the new wires to the old wires so that the splice is strong enough to be pulled through as well, but not too bulky to catch on things as its being pulled through..
You can get some wire snot/wire lubricant to smear on the new wire so it slides easier. You typically lay the wires next to each other and then tape them together. Let the wires overlap for like 6", then tape them together. Use a lot of tape. Start before the wires overlap, and cover beyond it. Use a lot of tape. No more tape.
The first time I was pulling network wire through conduit and saw the guy I was working with use what seemed to me like half the roll taping the wires together... I thought "Man that is a waste of tape." Sometimes I'd get away with it, but when it got stuck one time and I pulled it apart I learned the value of taping it like crazy.