I am a bit pressed for time, but I can address a few things quickly.
For me: I calculate everything, including fragrance and preservative, as part of the 100% . I use the batch calculator at Wholesale Supplies Plus, and it makes it very easy.
Your other questions can be best answered by reading a blog written by a woman named Susan in British Columbia. You can find it at this link:
http://swiftcraftymonkey.blogspot.ca/On the right hand side, there is a good listing of topics. She also has an E-Book specifically on lotion making that is available for a small donation to her youth work. It is WELL worth the money.
It may seem easier to get answers here than to do your own research, because yes, you need to do a LOT of reading to make sure you understand everything. I promise you, it is worth your time if you want to do this right.
Sometimes on these forums, people will see questions like yours and ignore them, because we may feel, rightly or not, that you are looking for easy answers instead of doing the research yourself. Or sometimes, it is because you asked stuff that would take hours to type out and explain. That is probably the case here.
good luck, and go read!