MacBook repeatedly reboots into previous boot modes

What might the details be of “following an incident… prompted to use a startup modifier”?

And the NVRAM and SMC reset and multiple (failed) macOS reinstallations imply either a hardware problem, or (more likely, for now) corrupted GUID GPT partitioning.

Which means erasure.

Get a Windows partition backup if you can.

You probably can’t get a macOS backup, so hopefully you had Time Machine running.

Here are the articles for the general sequence starting with erasing, booting Recovery on your particular Mac, and restoring your backups:

• Erase your Mac and reset it to factory settings - Apple Support
• How to reinstall macOS - Apple Support
• How to start up from macOS Recovery - Apple Support
• Restore your Mac from a backup - Apple Support

FinnedFrontier wrote:

Whenever my MacBook Pro attempts to boot into macOS, it almost instantly shuts down. However, if a startup modifier is applied, it attempts to apply it on subsequent startups. The startup volume selection function is the only modifier that is not copied during reboot. I have a Windows boot volume, which can still be reliably accessed and used without incident. A more detailed analysis follows:

Following an incident the past Thursday evening, my computer suffered an unusual condition: when prompted to use a startup modifier, it repeatedly applies it every time it boots, with the exception of the volume select control. Standard macOS recovery displays an Apple logo on following startups, but otherwise does nothing. Internet-based OS installation appears to exert effort, but continues to attempt reinstallation on following startups. Resetting NVRAM causes the startup sound to play, and does not help the issue. Attempts to start into safe mode or verbose mode do nothing. Target Disk Mode appears to work.

At the moment, I'm attempting another reinstall through Shift-Command-R.

Did you try from the Terminal.app to clear all boot arguments...copy and paste:

sudo nvram boot-args="" && reboot

(sudo note: your psswd will not echo on screen type it in anyway, then to proceed use the enter\return key.)