Accept my friendly advise even i'm not expert. Do some research on your own and use search. I'm beginner too and learned a lot just by trial and error. And using search. Your device won't explode by doing that. ( I guess. I take no responsibility if you succeed ) I hope someone guides you...
Flash it first, and tinker with your phone atleast one day and then go to https://faq.whatsapp.com/ if not satisfied. Do you think a random forum user can answer your question yes or no ? I say i don't effin know.
Useless to fix sms at this point as you are anyway going to flash stock. https://new.c.mi.com/global/ You flash same ROM you had before boot loader unlock. There is a good guidance.