qeustions on porting twrp image to new phone

I acquired an lg g3 on 24b. I figured I would make it as a spare incase I lost or damaged my original and use it to test newer versions like marshmallow or nouget.
Here's what I did
1. I I restored factory image using lg flash tool .tot version back to 10b
2. stump rooted it.
3. installed earliey twrp from 2014 with flashify
4. installed twrp 3.0.0
5. did a twrp back up including EFS
6. restored my jasmine9.1 -35b image to it. with EFS checked
7. Noticed my modem/baseband version was "Unknown"
I was going to flash the 35b modem but before that I had some questions.

Questions.
1. what exactly is EFS? ( am asking because I included it when I went from 10b to jasmine9.1 35b I need to know if that's something you dont flash from one phone to another)
2. if I flash my modem file to 35b wil I be able to flip flop back and forth and upwith modem files.?
3. how high can I go with revsions keeping twrp and modem/baseband without have to restore original with .tot if I want to get back to 35b.

I know enough to be dangerous so I'm asking cause I don't know enough! lol
thanks.

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