Turntable placement Advise.

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.

Get it out of the listening room. The acoustic energy produced by the speakers will excite the TT no matter where you put it in the listening room. Next best solution is an acoustic screen. I know, I know my solution is not practical, but you did ask.

Okay so let see if I have this right.
Hook up the TT to the phono pre-amp box using short cables. Then the pre-amp to the audio/processor which by the way it's an Amthem AVM 50 using the long interconnects.

Thanks