In your replies, I think you can just think of them as Debians
No, there is only 1 Linux distro that is Debian. There are many Linux distros,
most of us do not think of them as Debians,... Debian is Debian. Linux Mint is a
different Linux disribution, (distro), all the other distros are like wise different distributions, all though some claim to be based on Debian, they are no longer Debian, they have been modified and changed so much, Debian.org does not support them, nor is responsible for the packages for them, this is why the other distros all have their own repositories.
if i have fewer OS's, then i'm wasting my limited space with unused partitions.
This is not realistic. You can all ways delete/remove any partitions you do not need, and use the space to increase the sizes of the partitions you do use.
do you think the folder-based installs is risky, not possible with certain OS's
I have never heard of "folder installs", and I have never heard of puppy xtahr, or Stretch DebianDog , this entire topic is not about debian, but it is about grub, and various linux distros.
For now, puppy xtahr (another Debian-based distro) is the only one I know is going to styay with me, so I'll make that the master.
How to install grub2 from a Debian-based distro, over an existing grub installation that was installed by a different OS? Pardon my ignorance.
Questions about properly installing Grub2 or Grub, to other distros, probably would get more accurate answers if the question is asked on a support site for that particular distro. Also it should be noted, grub is not the only boot loader available, lilo might be a better option.
For example, if you were using Debian, but you are not, obviously this would be the correct forum to ask about how to install grub to Debian.
When you install Debian, the installer installs Grub2, and updates it, assuming you did the install correctly, and everything went well, the grub version that Debian installs, will be what is booted and it will have a menu where you can select any of the bootable partitions, regardless as to what distro they are.
Some of what is said in the OP , confuses me :
Debian is a frugal install, inside a folder on the Win 10 partition.
Are you really using Debian or not ? please be honest, and I don't mean some distro based on Debian, I mean Debian. I do not know anything about windows 10, nor have any interest, but I don't think Debian can run from some folder on a Windows system. I know Debian, Linux, and Ms Windows are kind of merging and getting more and more similar, but I don't think it has gotten that far, Debian is a OS, not some program, that comes as a windows .exe file, and you put it in a folder, expecting it to run.
==============
- I need to pick one OS as the long-term master OS for that partition (Mint wasn't it). For now, puppy xtahr (another Debian-based distro) is the only one I know is going to styay with me, so I'll make that the master.
Hopefully I can host multiple folder-based Linuxes in that partition for evaluation.
Ok, and I am sorry to be so long winded, there just seems to be so many questions in my mind, and this topic seems to have a lot of various distros, etc,... any way, finally, this will wrap up what I have to say :
I only have 1 partition, and 1 OS, that is "master", if you want to call it that,..
I use the term "host",... the 1 OS is host, and controls all of the others, I do experiment with many, Linux (various), BSD's, Minix, even a old Windows XP, and also FreeDos. So called "multi boot", and various partitions, Grub, etc, is a ROYAL pain, and headache, and yes after all said and done, it is limited to 4 or 5, bootable partitions, I forget exactly on that. There are ways to have more, but it really gets complicated, and messy.
Anyway, it sounds to me like you should really look at using a VM system, I like QEMU, but there is also, VirtualBox, and many others, maybe that is what you mean by " folder-based Linuxes ", ? Are these just virtual images, in folders, ? In a way that is what I do as well, I have each OS I am testing and working with in a Directory, which contains the image, and everything needed to start the VM,... My ram is rather limited, so that does limit how many VM's I can have running at the same time, but it does not limit how many VM's I can have, in other words, if need be I can shutdown 1 vm ,say (Debian-2), and then start another, maybe the Minix 3,...etc,...
I have 3 running now, all at the same time,... you can't do that with a multi boot system, But you can with VM's.
============================