Each password with any given seed will generate a completely new master public and private keys. Think of it as a completely new tree.
Now I'm a bit confused. I don't get it how master seed+password work. Can I recover coins with master seed if I forget the password?
No. If you use a password and forget it the coins are lost forever.
What if I disable the password and have only one PIN? If I forget the PIN can I recover the coins only with the master seed?
Yes, the PIN only protects the hardware on the device from being used without entering the PIN first. If you stop using the password and forget the PIN you can recover the coins from the seed. But be sure to send any coins in your password protected wallet out to another wallet before removing the password protection. Then after removing the password protection send the coins back to the Trezor.