1) Where should I stop a stretch? The tapes offer no help here. Do I stetch to where it pulls or just below that? I know I shouldn't be in intense pain but should I have any?
I would stop a stretch as soon as you feel a nice pull. Don't push too hard or you'll hurt yourself, but it you don't push at all, you won't progress. In yoga, you can use your breath to go deeper into the stretch. Slightly relax the stretch on the inhale and fall deeper into the stretch on the exhale. If you hold a position for several breaths you can slowly ease deeper into it without hurting yourself.
2) Is there any modified form of the 'plank' position that someone with NO upper body strenght and little ab strength can do? Or do I just keep attempting it?
Try the plank with your knees on the floor, kind of a like a "girl pushup." After you feel comfortable in the modified plank, lift up to your toes & hold the full plank for as long as you can, then put your knees back down.
3) How long should I do it? I have 4 20min. programs. Should I only do one program, or two?
I would start with one 20 minute program & gradually work your way up. Like anything, you do not want to over exert yourself in the beginning & risk injury.