Interesting questions and points, and I’ll address them as listed in your post:
1.) Prop 8 is constitutional because the States’ retain all power not prohibited to them by the 10th amendment. As for it’s being unconstitutional because the privileges and immunities clause protects life, liberty, and property is a bit off. From my understanding of the profoessors’ teachings, life meant not being killed, liberty meant not being enslaved or imprisoned, and property was, well, all forms of property. Considering marriage doesn’t fall into one of those three definitions, I think it is safe to say Prop 8 is very much constitutional and perfectly fine for a state to pass.
2.) While, yes, the Civil Rights Bill of 1866 did allow for people (former slaves) to enter into contracts, it did not limit the States from legislating on what were valid contracts. That isn’t explained very well, but let me give an example. Prostitution. In certain States prostitution is illegal, but one can argue that a woman or man can legally enter into a contract with another to be a prostitute. So, if a State can pass legislation on prostitution as being an illegal contract, then they are also able to pass one on marriage. However, I don’t personally think of marriage as a contract in the sense of a contract between peoples for exchanges of land, between employers and employees, etc.
3.) See number 2.
4.) Therefore I think Prop 8 is constitutional.
As for DOMA being constitutional, I think under the Constitution as ratified, all powers not given to the general government were retained by the States, so Congress has no authority to legislate in the area of marriage. I dont know all the details about DOMA, but I find it to be unconstitutional.