Sculpting Hand sculpting head: how to deal with the head cap?

I have a couple of different go-to soluions for the head cap.
The most straight forward is to simply sculpt the head in stages. It may go something like this:
1. Make the core with integrated place holders for the eyes.
2. Rough sculpt the under structure for the facial features (the "skull" so to speak), leaving the placeholders for the eyes clear. I usually also leave a bit uncovered at the back of the core to let air out. Bake.
Add clay to the face and sculpt the facial features. Shape the joint socket (I usually use a placeholder here as well). Score the parting line for the head back. Bake.
3. When the clay is still warm after baking, I carefully cut along the scoring line with a thin blade. I cut all the way into the core. Let cool and then remove any clay beyond the cut. Remove the core through the opeing that has now been created.
4. Now is a good time to fine tune everything that needs it. This is when the dremel comes out. This is the time to smooth out the inside, fix asymetry issues, bevel out the eye wells, make sure the neck joint fits and functions and all that less glamourous stuff. This is when any engineering around the join to the head back is done. If you want any kind of lip, edge, hook, sockets or anything like that, this is the time to do it.
5. To have something to sculpt the headback over I make a new core, one that fits into the front on the head. I sculpt the head back over this, gently remove it and place core and headback only into the oven. Bake.
6. When cooled I remove the core and do all the other steps such as smoothing the inside, adding any sockets, edges etc. to match the coresponding work done on the head part. This may take several rounds of tooling, adding clay and baking.
7. Finally I place the headback onto the head and fix it in place with a bit of modelling clay (make sure it's sulphur free if you intend to cast your head) and sand the joint smooth.
8. I add ears last, because I think it's easier to decide of placement and proportions when I see the entire head.

And that brings the head to a stage where it's ready for finishing and priming.

When I make my human tinies, I want them to share identical head backs, so for them I sculpt every new head with an existing head back as a template. So there are situations where your may want to develope a different process.

I'm not sure I understand your second question, but as for baking the clay over and over:
You can do that. Just make sure you don't over bake and plan your work so you bake somewhat even layers to avoid uneven baking. If you are not very experienced, or if you are working with a new type of clay or a new oven, make some tests to get a feel for timeing and temperature. The time and temperature given on the packageing is a guideline only, different ovens and different sculptures require different temperature and time. Don't trust it blindly and never leave it unsupervised.
If your design have thin parts that sticks out (elf ears, fingers etc.) save them for the last baking, if possible.
The time needed to fully bake something depends on it's thickness, so a thin layer added to a previously baked piece should be timed based on the thickness of the fresh clay, not the entire piece. This means that the bulk of the piece may barely have time to fully heat up, while the new addition bakes. But on the flip side, sometimes bulky parts or cores can work as heat sinks, preventing newly added clay form reaching the temperature it needs. This is one of those things that you work up a feel for with time.
Do not over bake. When polymer clay burns it gives off very toxic gasses. If a part of your sculpt is starting to turn dark, turn off the oven right away.