There are a number of things holding up preventing my agreeing:
1) FIL never turned over discovery.
Generally problematic, though you can work around this by demanding that he prove your numbers are unreasonable with actual evidence. I can see how this would slow down the process.
2) FIL wants me to give him gifts that were given to me. Can't use the word I want. Imagine it.
What do you mean by this? He's asking for some of the items that were gifts to you (by whom?) in the settlement?
Gifts exclusively give to you outside the marriage are considered non-marital property; gifts within the marriage (say, from your husband to you) are considered marital property and fair game for dividing in a settlement. However, you might consider giving up the gifts regardless of source if they are less important to you than coming to a favorable agreement. Focus on the end game, not how you feel about any particular negotiation point.
Mediation isn't fun, it won't make you feel good, but you will only hurt yourself if you let those feelings drive your negotiation position.
3) FIL wants non disparagement clause but refuses to give us the clause, so that I can determine if it's too restrictive to sign. In the meantime, however, I'm turning the past 12 years into fiction.
This doesn't seem like a problem. You have a lawyer - have your lawyer write up the non-disparagement clause and submit it.