Pressure on case beside trackpad leads to cursor movement.

It's most likely your battery. Try taking out the battery, and using your computer plugged in.

You say that you're using a 3rd party battery, it may be just slightly off in design, and pushing up on your trackpad.

Another possibility is that your bottom case is slightly bent. If the center is slightly higher than the sides (frowning) it will cause this. You may need to do some 'manual adjusting' (bend it back into place). Do this with the battery out.

Check the 2 cables behind your battery! apply some presure on them and watch your curser. theese cables tend to cause problem of different kinds.

I had that problem for a long time!

it is because the keyboard/trackpad cable is torn. i fixed it by appleing pressure on the cable!

I had a similar problem where my button wouldn't click, and the case felt warped.

If I were to press the bottom case so it sat flat on the desk, the cursor would select areas of the screen like I was pressing and holding the trackpad's button and then moving the cursor.

It turns out that my battery had swollen up like a balloon in one spot. I almost had to pry it out of the case.

Now when I use the computer plugged in without a battery in place, the trackpad and button work just like I remember them.