Ota After Bootloader Unlock

Dears,
I'm going to unlock the bootloader of my Mi5s in order to install the last Xiaomi.eu ROM.
I asked the permission to xiaomi and I received the confirmation. I'm installing TWRP recovery and then I'll flash the xiaomi.eu rom.
I have a doubt.
If in the future I want to come back to original rom,
1. can I flash original rom by TWRP?
2. The OTA will work?
3. Can I lock the bootloader by Fastboot or is it no more possible?

thanks!