And if I import my wallet into Mycelium through a trezor, will the wallet developers not get my private keys, can I somehow get them from the trezor using third-party software like this wallet?
No.
Your private keys never leave the device. You can not access them in any way (except for vulnerability which might exist, but are not known yet).
A hardware wallet keeps your private keys secured and only uses them to sign transactions/messages. The signing process is completely done within the device.
It just has one interface which accepts unsigned (or partially signed) transactions and gives back a signed one.