@Chrysostomus, I'm using m-a "intensively", for my installation and also to install PCs of other people: its amazing and very useful!
I send you some considerations and I hope that them can be interesting
I'm using both the m-a iso and a dedicated manjaro installation with m-a on board; in my opinion the second is the best choice..
1) also if I change pcman-mirrors.conf every time it restart with the previous configuration: very annoying. The default mirrors are slow
2) if I repeat the installation, with the same M-A ISO USB, every time it download everything: no cache
3) When I use m-a without redefine the destination HD partition schema from the scratch, but mounting existing partitions, without formatting them, I get an Error  (I posted the screenshot previously) and no way to install. It happen before to install the "base package". I don't know why and till now I wasn't able to find a trace to follow it, by now, but I continue to test.
3bis) If I repeat the mounting process, the installation go ahead, but it isn't t done on the target HD, but... somewhere else.
4) M-A ISO is great cause you don't need to take with you a PC, but a simple USB Key. Shall be a lot more usable, IMO, if it was possible to get the previous functionalities pre-configured
M-A DEDICATED Standard Manjaro installation
1) I can configure the M-A pacman-mirrors and other .conf file as I prefer: very very comfortable
2) When I repeat an installation on a different device, m-a download ONLY the deltas: another very useful functionality
3) The "Error: " is always present and with the same destination device the behaviour is the same.
I use btrfs and I like how you are thinking to implement the subvolume.
Usually I use encrypted partition (luks). I must try to use different encryption level (root, home) for people that want a similar approach and don't like to use btrfs or lvm (I'll send you a feedback) People sometime is strange...
Great work! In my opinion the best installer in the Linux environment and not only!