Day 2 - Rear Support posterior pelvic tilt

Should I do a posterior pelvic tilt during the Rear Support position?
I've noticed that sometimes my belly goes higher than my chest, but I'm not fat at all. I wonder if it is posture related.

Yes, you should. Try to maintain a straight line from shoulders through to toes, and retract the scapula together

I got pain in my lower back/disc from Day 1's exercises and especially the crab. I also experiences some pain after the rear support position.
I've had problems for quite a while with this and it all started after a high fall when I was six. I know I'm walking around with an anterior pelvic tilt and often when I do back work I'm hanging by the disc so to speak. What do you recommend I do coach, besides following and being very thorough with the program? 
Also, is it okay for me to start experimenting with the Jefferson curl? I'm thinking a couple of pounds at most. 

Get through Fundamentals and then start Foundation and the stretch series. Just be careful not to exercise in pain so that you do not aggravate the back.

Please keep in mind the difference between posterior pelvic tilt and shoving the hips forward! Similar to protraction vs. jamming the shoulders forward.

It may be that in supine variations with PPT you are able to achieve a nice line without irritating the back, without yet being quite stable enough to achieve it in support. Spines are not to messed with; err on the side of caution until it's easy

How should I rest my head in the prone position. Should my forehead or chin be resting on the floor, or does it matter? Thanks!

As you are exploring the different positions you can vary from chin to forehead. When you do this compare where the different tensions are in the body. You should be able to do both head positions easily.