As far as I know, technically it's possible to transfer only full amount of funds from the address, not a partial one.
You're wrong.
So when only a part is withdrawn, the change is sent to another BTC address.
Depends on your preferences. You can switch this off by doing the following steps: Tools > Preferences > Go to "Transactions" tab > Remove the check on "Use change addresses". By doing this, the excess will be sent back to the originated address.
The question is: if I try to send a part of the amount from my imported BTC address, will Electrum pull the additional change address into the wallet automatically? Or will my change just be lost forever in this case?
No, it won't be lost.
Take note: Don't forget to back up the wallet itself (just in case).