Here is a suggestion:
If you can find someone who has --already-- flashed this particular ROM, you can ask them to take a TWRP backup of JUST boot (it won't take long, and it won't be that big). You can flash the backup from within TWRP.
It shouldn't hurt to be on an older baseband (modem), so that could get you going until you can access a PC.
If you intend to use a flashable zip in the future, make sure it includes BOTH /system AND /boot.
Sorry I didn't check on that earlier....it sounds like you had already tried to flash it though. Hope you aren't stranded in a non-working state right now though. If you can get your hands on a friend's PC for 15 minutes or so, that would be best.
Fortunately the only issue I have had a problem with has been it constantly failing so thank you for not only explaining why but actually responding and not telling me to "keep reading" which happens alot here.