[OnePlus 7T Pro (Regular & McLaren) Q & A] Artimis:
I cross-posted this in the General Android Questions forum too but have not had any responses

I can't seem to find any credible information about how to prevent a device from cutting over to the inactive slot on reboot in the event someone intentionally doesn't want to proceed with the update process.

Specific Scenario

I am running a OnePlus 7T Pro McLaren 5G (on VZW - not that it matters) with an unlocked bootloader and Magisk/TWRP installed. It's been running great on Android 11 and I'm finally comfortable with the setup.

Unlocking the bootloader wiped out reserve.img (which I never restored) so I've never been able to take an OTA update as pre-verification checks have always failed. I'm good with that for now (and also disabled automatic updates).

Somehow, the OOS 12 (Android 12) update automatically downloaded and installed!! I now have until 3/26 before my phone attempts to reboot automatically to complete the install which I don't want.

In order to ensure I don't lose root, etc. I already flashed the TWRP A/B retention script and used Magisk's "Install to inactive slot" option.....which indicates it has updated my active slot (b) to the inactive slot (a). In hindsight, I'm not positive this was the best move but I've also heard that patching boot.img using this option may also result in OTA's failing to complete the install process thus reverting back to the current active slot (which is what I want).

General Questions

My questions now are more general for those more intimately familiar with A/B slot partitioning, Magisk, and TWRP. In order to proactively remain on my current setup (slot b), etc. I've thought of the following options and would like some input:

1) If I run Magisk's "Install to inactive slot" option again, will that effectively undo the previous change and keep onto to the current slot?

I ask because the output when running this command the first time indicates it is identifying the active slot (b) and then switching the active slot (b) to the inactive slot (a). If I run it again, will it switch the active slot back to "b" on the next reboot? Or will it still report my current slot (b) as the active slot and just essentially re-run the same change?

2) Should I reboot into fastboot right now and then use the fastboot setactive command to switch to b?

If I do this, would the initial "reboot bootloader" command boot to a or b? If it boots to "a" what happens if the boot fails due to some incompatibility with the TWRP retention script (or some incompatibility between the TWRP version I flashed and Android 12), would it know to failsafe back over to slot "b" since the boot failed?

3) Should I reboot to recovery and use the TWRP option to set the slot to b?

Same concern as #2 applies here. I've also seen a lot of warnings never really to use this option in TWRP as it messes with the calls to mark boot successful that the A/B protocols rely on. Only reason I'd consider this over #2 is it can all be done on the phone (not really the most compelling reason :) ).

4) Should I simply reboot and cross my fingers that the update fails either due to an issue with TWRP or validation check against the modified boot.img and defaults back to slot "b"?

The risk with this one is if all is successful I'll be upgraded to Android 12 but at least I'll keep root.

Any insight would be much appreciated.