"I have a Delta single control shower valve, but the hot and cold seem to be backwards (the cold is on the left and the hot is on the right). Is there a solution for this other than tearing open the wall and reversing the pipes?"
We have the solution for you!
Reverse ball for single acrylic knob: order the #DE-RP16423 = $29.49

"How do I get the seal out after I remove the cartridge/ball?"
Push a pen or pencil into the seal then pull it out.

"Are there any helpful hints for changing balls/cartridges/seals?"
When changing the ball/cartridge and/or seal it is recommended that with an assistant, you hold a cup or towel over the open valve, turn on the water to flush out the lines, then install the new part. This may help to extend the life of the parts.

"I put in a new cartridge but now it turns on and off opposite of before. Did I purchase the wrong cartridge?"
No, you will need to remove the cartridge and turn it 180° then put it back in.