Dear AA, cool info! This 'little brother' is 100 years old, BTW!
So, if I get this right (paying no attention to the iPod angle, Monterey was installed resulting in two disk icons on the Desktop. Good so far?
The way you resolved this was by booting from 10.12 (Sierra clone?) & repartitioning as MacOs Extended (Journaled). Right?
Perhaps others more geeky than I can weigh in. Apple is pushing for APFS, despite the difficulties with applications, external disks & backups. I mean, eventually these will no doubt be resolved. But I'm just not an 'early' adopter & the old OS works fine.
From what I understand from users is Big Sur & Catalina were Frankenstein OSs. But I look at Monterey & Ventura & not of those whoop-te-do 'features' hold the slightest part of my interest.
Wifey's stuck with Monterey & its two disk icons but she's not married to her Mac in the same way I am, 15-16 hours a day. (I should get a life, eh, or at least some work-life balance!)
My little brother is only 45 years old. LOL he does read this EhMac site, even though he is not a member here. he knows how much i love my Apple computers. =)
"Monterey was installed resulting in two disk icons on the Desktop. Good so far?" if this was installed by Apple, pre-installed, they would of done it with 2 partitions. under APFS format in which i am not a big fan of. like you mentioned earlier, APFS is not really stable under my kind of use as i do a lot of back ups and APFS corrupts my data when it is time to encrypt my drive which creates "automatically" a so called container under the APFS format.
when it comes to boot from a 10.12? you can use whatever OS that your mac can boot from and use disk utility and erase the drive and create a new partition in Journaled extended if i am not mistaken. then you can re-install your os wether it is from a back up or a cloned drive. i usually use Carbon Copy for cloning my original drive in case i get a HDD failure so i don't have to waste an entire week end re-installing my Apps, passwords, web site favourite and .... etc.....
i'm not a big fan either of the new OS. for what i am doing anyway? i don't need to upgrade to the latest as all my newer machines uses High Sierra on my 2015 macbook and 2009 MacPro 12 core. it has been pretty solid so far and trouble free for all my apps, but all depends of what you do with it, but i mostly do gaming, web surfing and some 3D graphics and some video editing
When it comes to have a life? ......... you have a life, you are not the only one. i don't have a wife, no kids, so everything i do? is in my time, my life and i love fixing things like old Apple computers. i also need a life as i collect older Mac and one in particular is a Apple LISA, TAM and some Ipods to complete my collection.
You have a life. =)