I am not a professional, but I believe that there is no problem for Trezor firmware and Trezor Suite, only the hardware itself may have problems. If a backdoor is left on a small amount of hardware, ordinary users and professionals cannot detect it.
You have said yourself that you are a newbie, so what are you basing your assumptions on? Simple believing it isn't enough.
They released four different backplates... Apart from the cosmic black, you can choose the stellar silver and solar gold as well.
Yes, you are correct. The universal model comes with 4 different back covers. You forgot to mention the pink one as well. So, the total count is 5 back covers, 4 for the Universal model and the orange one for the Bitcoin-only Trezor.