I don't know that much & may be misunderstanding what is happening or calling things by the wrong name. I'll try explaining in more detail. OP directions are in italics...
-- after getting into fastboot I:
"open a command prompt and type "fastboot flash recovery <path_to_recovery.img>” without the quotation marks."
The cmd prompt always indicated that it worked (msg: "sending recovery Okay, writing recovery Okay, finished"). At this point, I thought the TWRP recovery was flashed, and the next part would be accessing TWRP in order to wipe/format (& then flash the 2 zip files). So the next step is:
"unplug the usb cable and power it off by taking out the battery and putting it back in."
Next comes the problem part. I must be doing something wrong. I thought the purpose here is to access TWRP.
The OP says:
"HOLD VOLUME DOWN + POWER until you see the LG logo and then temporarily release the POWER button, immediately pressing and holding it back down until you see the white prompt screen."
At this point, one of 2 things happens
1) I get a white screen that says "Factory data reset". Do I want FR? Yes/No. It requires answer to move on, but entering yes/no requires releasing Vol down button. The next screen tells me to relock bootloader. Then it goes to LG logo screen & factory reset.
OR
2) A couple of times, Team Win screen opened. (Don't know what I did different.) Some script went flowing through, then it quickly sent me to the LG logo screen & factory reset.
Major confusion at this point. OP says the [volume down + power] buttons can only be used the first time. Then it gives alternatives to boot into TWRP either by using app that requires root (unrooted, so not an option),
or:
"u can use adb command type [ adb reboot recovery ] in adb terminal to boot into twrp".
I went into fastboot & entered command. Always got msg "no devices/emulators found." I've tried several times. So I can't boot into TWRP in order to flash the no-verity & SuperSU zips.
I've repeated these steps several times. Am I reading it wrong?