Setup Speakers Along Long Wall?

Answer to long versus short interconnects versus speaker cables. Long interconnects are okay if you have high impedance system. Balanced helps but is a desperate measure And, you don't have a high impedance system. Consumer cables are 3x interconnect and 10x speaker cable impedance typically. This is wacko impedance but that's the market. Therefore short interconnects are always indicated in audio systems but try it yourself and you will see

Answer to long versus short wall. More important is room symmetry and acoustic insulation.  Then you mentioned the feedback due to standing wave room residence on your turntable. That is hit and miss and I'm afraid you're on your own with trial and error about that. As a beginning remedial measure I would insulate the entire wall and sides behind your speakers for a live end dead end effect.

I appreciate and will try to take the above advice in that it makes sense to me.

The balanced phono preamp that I was looking at isn't available until November, so I'm going to buy more bass traps, lower my turntable rack and see if I can keep my interconnects short.

Recording studios will sometimes use long interconnects without worry. I've had to deal with all the size shape and position issues you mentioned and cable length has never made a noticeable difference. Speaker cable length also didn't change sound (using 14 or 12 awg). Booming sounds more like room nodes. When you say more bass traps, are you using thick insulation style traps? Have you tried moving the speakers one or more feet in each direction? Cheap tweeks to try first!

15 foot speaker cables aren't a problem. I use them with no ill effects.