CYP, Sorry she made the wrong choice. How awful for you.
It sounds like you are dealing with this as well as can be expected. As others have said, there is little value in asking the Elders to sign a paper saying something is ok. Even if they would (doubtful), it's not a contract, so they can (and probably will) break their word at any time.
However, what you could try is to make a list of the things you want to do with your family, find bona fide WTS literature that says it's ok, and meet with your wife and the Elders to discuss. Show them the articles and ask if doing any of those things would constitute spiritual endangerment or whatever they call it. Make sure to ask your wife, in the presence of the Elders, if any of those things would violate her conscience. (Examples: telling your wife to vote, telling your wife to cook holiday dinner, hanging up holiday decorations in your home, etc. These are things allowed, according to the literature.)
Take notes as to what they say (or heh, heh, conceal a voice-activated audiotape on yourself). Then if they break their word, at least your wife would know the Elders were breaking their word.
If your wife ever does meet w/the Elders to claim spiritual endangerment, you would have documentation of the discussion. You could also try to request a meeting w/the Elders to dispute the spiritual endangerment charge.