Is it possible to have pixelated graphics on a MacBook Pro due to a defunct trackpad?
It's very common for one component it's connected electrically to another component to cause noise and other issues. Whether it's the most likely cause is debatable, but Apple's service centers have direct access to engineering and a huge wealth of repair data so they would be my first authority to go to determine what is likely and what is not.

Without the detailed diagnostics from your specific repairs as well as the actual service manuals it's hard to second-guess Apple.

Apple provides a nice (IMO) warranty of 90 days and any repair, so it's hard to see how they would be scamming you.  If you set down and catalog each interaction with AppleCare, have you spoken with them 10 or 20 times? 

Also you need to collect all of the written estimates and repair details to know what components have and have not been examined by a technician. 

The only time I've known Apple executive relations to be unable to resolve A customer issue is when Apple has already bent over backwards to accommodate their responsibilities and person requesting the repair is the one being unreasonable.  I certainly wouldn't accuse you of that with the little data presented, but if you really still unsure collect the facts and data you have and give it to someone who is somewhat impartial and have them give you their opinion.

Also, a final note - unless you are running stock OS or an apple benchmark, I've seen all sorts of gfx card mods and graphics tools cause exactly what you see. My guess is Apple knows other parts to be good and is guessing the trackpad is the most likely of the ones that remain in tested. 

I would ask them the cost of a depot repair, and you'll have to be Ok with paying for it if they replace parts - even if the benchmark fails to run. If your Mac boots the normal OS fine, I would not pursue further repairs till the situation deteriorates.