Hi John, I did the same you are asking for and here are my coments (sorry about my english, I'm just trying to help you):
I tried to use clear on clear parts and did not work. The clear part can be melted and also is not transparent anymore..a little points (like dust) came on and not posible to take it off.
By the other hand, to protect decals on clear parts, I use tamiya clear (X-22) very thinned and this works. Airbrushed with low psi.
Regarding you bonus question, I did not recomend to use clear over parts cover with future. The future is "greasy" and will be dificult for the clear to cover the parts. Hard to explain in English but if you do this, is like apply paint over a "wet" model.
but, the future can be easily remove it with liquid to clear windows (you can get this at any grocery store) the same that you use at home.
Just deep the part/model on this for a couple of minutes and then use water and that's it. I did this on a body model with no decals, but was completily painted and polished and works OK. Even I use the same technique on plastic clear parts covered with future and works too.
I don't know if this clear windows liquid "attack" the decals, I never did this before.
I hope you can understand my poor english
and also can be useful fo you.
Regards