You've heard right, my friend. White enamel (of EACH and EVERY given brand) has a tendency to turn to yellow after a while. So, no matter what brand of white enamel you use, you are meant to end up with "yellowish white" after all... On the other hand, ACRYLIC white never turns into yellow, hence I believe that your options on the subject are pretty much obvious.
The yellowing effect on clear coat enamels is mostly brand dependant, so you'll probably have to ask your way around. I had no "yellowing" problem with Humbrol clear coat (flat or glossy) whatsoever, so I can tell you this much.
Now, I'd never -ever- spray enamel over Future. Doing so, is like inviting disaster into play... You can find more info on this in the <Strange paint behavior> post and in my recent reply in the relevant thread, just a little below in this forum...
I would dare to spray acrylic coat over Future if I had no other option left (enamel is out of the question, as stated above), but in any case I would try to remove Future first, just to be on the safe side... You can find info on how to remove Future in many posts within this forum.
Wish you luck...