Didier:
Of course, not all Calvinists believe in double-predestination. This is often the debate that I and DeputyDog have, wherein he believes in it, while I do not. Hence I describe myself as a moderate, rather than a high- or hyper- Calvinist.
Sirona:
As you note, many of these things are not mutually exclusive. It is quite possible to be a monotheist / polytheist / deist and also a panentheist. It is also quite possible to believe in reincarnation and predestination with no contradiction.
The freewill debate is approached from the view of there being two sides to the coin - God's view and Man's. From our perspective we have complete freewill. From God's perspective He allegedly can see the end from the beginning and nothing happens that He doesn't know about. It is all predestined. The two sides appear contradictory if you attempt to view them from the same vantage point, but once you realise its merely a matter of perspective it seems rational enough.
The issue of sin is little different. Sins have been paid for by a lamb that was slain before the founding of the world, and yet it was also an event that was carried out "in" time. If you're a universalist (believing in a universal salvation for all mankind) then there's no unfairness to account for, aside from the crappy providence that many experience in this "hell on earth". If you're not then you have a little explaining to do, but can still come up witha fairly plausible explanation for the diverse opinions found in the canon of 66 books.
Following on from the "lamb" comment, and to fuirther address your other comment: An eternal "Son of God" threw himself at "death" and allegedly triumphed, but not without repercussions.