I got the laf_a and laf_b that ngowner provided here on page 12. I see they are 20e pie but they've worked for me. I went between flashing v35 eng. and deleted laf-a and b and reflashing them a number of times to try to flash twrp but no success. You're right I have fastboot when I delete them but then I can't flash anything because they're deleted ...So you're on Sprint 20g, which is Android Pie. First (and always) you need to use qfil to backup any partitions you plan on modifying. You say 'someone here provided laf backup' that you used to restore, but that's a prob because Sprint doesn't make their kdz available and I've not seen any backups for the Sprint firmware... so what laf backup did you use?
And, as I explained, there are 2 versions of fastboot; 1) v35 eng abl and 2) laf nuked (watered down) version of fastboot.
I've no experience with the Sprint (watered down) version of (laf nuked) fastboot, but it seems to be like other mfg versions, in that it won't flash a partition. So, you have to use the v35 eng abl fastboot to do that.
Unfortunately, the v35 eng abl won't 'boot' a partition unless it's Oreo. You'll have to use qfil and save your boot partition (a or b), and post it here. I'll inject twrp into it for you and post it back, then use qfil to put it back.
Here are the boot partitions. I hope I did it right? I'm not experienced with QFIL. So I went to tools -> partition manager -> boot_a -> read. Is that the way?