I'm pretty new at this too, but I've been using an RC-20 for a couple months. I can give you a few answers.
>If you use an extra pedal to move between loops, does it move at the end of the loop, or whenever you hit the button?
The extra pedal deal is not that great. You can use the pedal to move forward to the next loop in the sequence--not backward, not skipping loops, just forward to the next loop. Say I'm playing loop 1; at any point while loop 1 is playing, I press the pedal, and when loop 1 is done, loop 2 starts playing. If I press it again, when loop 2 is done playing, loop 3 will start. You're really better off bending down and turning the knob to the next loop you want, if you want to skip, say, from #1 to #3.
The pedal also lets you use the reverse function, but if you're not into reverse, then it's not that great.
> To delete old loops, start new ones, and move between them, in a live setting ... undo feature:
To delete an old loop, you stop the loop, then hold the pedal down for 2 seconds. if you haven't saved the loop in the memory, this will delete it. If you saved it, then you have to bend down and press the write and edit buttons down for a couple seconds. Then straighten up and start looping again. On-the-fly pain in the buns. You can't undo a part of a loop, only delete a whole loop. Then, you turn the dial to the next loop and either start from scratch or have something already saved in memory, like a basic bassline, that you can build up from. It's definitely not as flexible as, say, the Repeater. I've been trying to work it out so that I can really loop on the fly with the RC-20, but it's a bit--as they say--wack. I love it for songwriting and sound collage work, though!