Nandroid backups are all about strategy.
What you care about is what to do if you need to do a restore.
Let me kinda answer your question in reverse of how you asked - I'm pretty sure it'll be clearer that way.
TWRP won't recover a backup unless it recognizes its own structure.
The structure looks like this -
Internal storage (the root of it iow)
--- TWRP (folder)
------- numbers+letters (folder, note a)
----------- name-or-date (folder, note b)
Note a - that's your serial number, leave it alone.
Note b - what's inside is the nandroid backup, stay out of there.
Now, make a folder on your pc -
Nandroids
Now, every time you backup your nandroid to your pc, copy the TWRP folder into Nandroids.
After the first time it will look like this -
Nandroids
--- TWRP
------- numbers+letters (your serial #)
----------- name-or-date
And over time, it will look like this -
Nandroids
--- TWRP
------- numbers+letters (your serial #)
----------- name-or-date
----------- next-name-or-date
----------- another-name-or-date
----------- and-so-forth
When your pc backup is done, leave the STRUCTURE on your phone, just take away the nandroid. It'll look like this -
Internal storage (the root of it iow)
--- TWRP (folder)
------- numbers+letters (your serial #)
That will cost you almost zero bytes to keep the structure if you ever need to have it ready to pull back a nandroid.
In case of catastrophe, and you can't get to internal storage, your pc will have the structure you'll need for an OTG-USB nandroid recovery and you'll do that like this -
Root of usb stick
--- TWRP
------- numbers+letters (your serial #)
----------- name-or-date-of-nandroid-u-want
You don't have to be a pack rat and horde every little time you try a nandroid.
But if you have enough pc space, you want to keep a little more history than you think.
There's nothing like discovering that a nandroid was corrupted - it happens, wires, PCs, who knows - and just not caring because the previous nandroid is there and it's close enough.
Or you go, "Hey, wait a minute, I don't think I always had this bug!" And no big deal, you can reach back in time a month or two to check.
Nandroid names are not sacred.
They're just folder names.
You can rename the nandroid name-or-date folder on your pc. "my favorite" or "best before new whatever" no problem.
When you go to restore, TWRP doesn't know or care about the original name.
Just stay out of the nandroid backup folders.
You can be curious and you can look.
That's all until much, much later and you're an expert.
In five years, I've had to look inside a nandroid twice. Once was to show someone what he needed. The other time was a mistake - the nandroid was fine and I had no business there.
Let me know if this makes sense or what doesn't.
I'll answer your other questions after this.
I know you're anxious and I am happy because you have good reason to be.
But trust me, total nandroid Nirvana is your friend before anything else.
Questions?
