When is the latest I can apply for FLR(m)? Is it the day my visa expires? Or the 28 days before?
The day it expires.
You can apply for FLR(M) anytime between the day you are married (as long as you have your marriage certificate to include with the application) and the expiry date of the visa.
So, if you are applying in person, your appointment must be on or before the expiry date.
The 28-days before expiry part only applies to applying for your second FLR(M) visa and for ILR, because you won't meet the residency requirements for them until 28 days before expiry, but there is no residency requirement on a fiance visa.
Also, as I am changing my last name and renewing my passport to reflect that, obviously I will be filling out the application with my married name, I just seem to be confused about the whole application in general because of my name change... Like will it be more difficult for some reason? Idk, I'm just worrying about it all lol
No, not at all. Your visa is only issued in the name on your passport, so it doesn't really matter which name it is in, as long as your passport and new visa match.
It can actually make things easier if you change your name on your passport before you apply for FLR(M).... because then you can use your married name for everything right from the start.
If you didn't change your name on your passport, then your FLR(M) would be issued in your maiden name and you would have to always use the maiden name for travel and future visas, while using your married name for everything else.