Setup Speakers Along Long Wall - room with sloping ceilings

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Your question no 2:

Unfortunately, I cannot see your pics.

Conditions in a room with slope ceilings are far from ideal. Initially, I had my speakers placed in front and under of one slope side. SQ was very bad. So I changed the speaker position to the front end wall. After some meticulous position work respecting the accoustics of the room, I am now satisfied with SQ. It turned out that nearfield listening is the best way to follow in such room. As good as it gets.. Although, Big Band Sound and Classical Orchestra Sound is poor (relatively low ceiling heights, see pics)

Answer to your second question: with speakers on long wall in 12 ft wide room, your back ends up against the wall. Basically impossible to get any control on behind your head reflections that way. Diffusers won't work that close behind you. Ideally need 8 ft or more behind you to get the best sound. Just my experience. I would stick with the short wall. 

That’s how my Tannoys are set up, along the long wall...had no choice. Luckily they sound good there. I still have about 5 or 6 feet behind my couch before the back wall. 

With the potential cost of good cables, I might first look into what’s going on w/ your turntable & why there’s a problem ? I use that cartridge but not familiar w/ the Technics. My Basis table / Vector arm is likely substantially better than it but it’s pretty close to to my speakers which I play loudly & have no issues. There are many fine, not crazy expensive turntables now w/ good suspensions that might work well. There are also many types of “after market” fixes to put your turntable on that might work too. I’ll guess there’s a thread on this site that addresses this. Just a thought. 

Long interconnects are always better but over 15 feet should be balanced.

NEVER put a listening position against a wall. Somewhere within the middle 3rd of the room is best.