Unlike the front shocks, the ones for the rear are not labeled left or right. I don't think it matters in this case.

I didn't do that. However, as I've mentioned, I've swapped the rear shocks left to right. If one shock has more stiction, more compression, or is defective, bent, contains marshmallows, made in China, or what have you, then the car would have been lopsided the other way.

The problem is no matter what I do, the rear passenger corner always end up higher than the rear driver corner.

 

I read a post on a ricer forum about adjusting coilovers, which I thought would solve the problem. So I tried it.

I disconnected the rear sway bar from the drop links on both sides and unbolted the bottom mount of the rear shocks. Then, I adjusted both shocks to about midway--the post said that it is better to lower than raise when attempting to level the car. I put everything back together, except the sway bar and the drop links. The idea is to prevent weight transfer from affecting the adjustments.

No dice.

Still the rear driver corner had to be kept much much higher than the rear passenger side. Unevenness persists.

 

Have you been changing both the spring and strut from side to side? It might just be a spring that's not right. Measure the springs side by side and see if they are the same height unloaded. If they measure differently then that might be problem. If you spoke to H&R and told them "I have 1 spring that is X high and another that is Y high", then they might be more likely to listen to you and give you a new spring. All else being equal, it's usually the spring that determines the ride height. If the springs are the same height, then you might have one that has a different stiffness.

 

Meh.

Not sure if I want to go through all that trouble after what I had to go through. If indeed the problem was uneven springs, I don't think I would accept just a replacement from H&R. They better send techs to my garage to install a brand new set for free!

Plus, what if after replacing a spring, the heights are still uneven. Then what? Suspect the frame of a brand new car?

Also, I just lowered all 4 coilovers down to the lowest setting and my MCS appears level enough.

 

In another thread with a similar problem, the guy went for a drive with 3 buddies in the car and it leveled out. It may just take a good load to get things to level out.

 

 

Sounds like you have your problem pretty well fixed now.

Not sure how everyone is doing their measuring.
I've heard of people measuring straight up the center of the wheel to the black fender trim which on the 1st Gen cars could have some play in how they are mounted..(Not sure about the 2nd Gen cars) One side could easily measure differently from the other.
Not inches but 1/8''-1/4'' maybe.. and yes a new car could not be sitting 100% level to start with, of course by putting coilovers on your replacing
what would usually cause that.. and once lowered it much easier to see a difference in the wheel gap to notice one side being off some..When you have 2 or 3 inches of wheel gap you'll not notice a 1/4'' difference..