I can understand the angst. Its hard to make a decision because we don't get any solidified answers, one doc hears of good results of one study another doc is watching a different one. Then they don't know if preventing the gvhd comes at the risk of overall outcomes or long term outcomes. It doesn't help knowing that 10/10 matches be it sibling or unrelated as you've seen first hand isn't a guarantee. On the other hand 9/10 matches work in many cases, and many 10/10 matches are successful. I read about Cytoxan but not enough were I found anything conclusive about it. Sorry to say these procedures come with risks so the game becomes can I get my odds better. The transplant docs know that a 21 year old has better odds than a 50 year old and so they may be inclined to encourage going ahead with it.
This link describes the 2 types of gvhd (acute & chronic) the grades, what its like, most of the questions you were asking.
http://en.wikipedia.org/wiki/Graft-versus-host_disease
Take Care and hang tough, Steve