Turntable placement Advise.

I would rather have the 18 foot run from phono preamp to system preamp than expose my TT to that much vibration. Bad idea.

Well you can't put it on a speaker so it'll have to go 18 feet away. How about a phono amp beside it to drive the cable? Or wall mount it to the side of the center speaker. ..

Agree. Either wall mount it or buy a stand-alone table from IKEA or maybe 2nd hand. Then use isolation under the TT. Also you'll need a shielded phono/RCA cable for a longer run.

You will not be watching/listening to HT explosions while also spinning LPs, will you? In other words, you will never have the center channel speaker on while using your turntable. If that is the case, I would not worry too much. Why not just try the TT in both positions are hear which sounds better.

Personally, I would not have HT and 2-channel audio in the same room. I would also place the turntable on a side wall on a very sturdy rack with some kind of isolation platform between the turntable and rack. But that is only if you want to optimize each of the two systems. Combining them as you describe may be a necessary compromise which you can't avoid.