I came back to report my findings.
I flashed Open Beta 18 firmware+modem on my LineageOS 14.1 installation. The flash did not break anything (just like others said). Also, the battery lasts a lot longer after flashing the package. I went from 2 h...
which build i should download? trltespr - trltevzw - trltexx ?? (i have a stock v 5 andriod version, this is the first time i'm going to change the rom)
VZW is for verizon, SPR is for sprint, XX anything else. I had an N910F, so I chose trltexx....
No, yes, yes.
You should keep your firmware updated to get a few of the system changes OnePlus introduces in their official updates.
AFAIK, LineageOS already has built in FW, so no need to flash FW if u using LineageOS (just like PA)