Ah, there's the rub
@Julien 
AAPL is Darwin, which is BSD, which is unix-y; so, yes: you can definitely/technically link a connected External resource to an Internal.
As long as the External is decidedly connected to the comp during the logon interval, you should not face undue hardship.
If you choose to move key Accounts to externals, the wise choice is to create and always retain an Admin account locally . . . it's not a matter of
if things will go Pear Shaped: it's a matter of
when
AAPL is no more (and no less) lazy than any other Distribution; non-standard configurations (<=10% Use Case Scenarios) are just not nearly as supported as the other 90% . . . not because any of the Distros don't care, but because the question just doesn't arise enough to warrant appreciable consideration.
AAPL, specifically, is more a One Device <-> One Use type of Company, so this idea just doesn't really enter-into-it all that often.
I've gone your intended route a few times over the years (mostly in the linux-y space), but the increased management was more than I found acceptable. I just get the Storage I understand I need, and roll from there.
I've never successfully used MA, so I can't speak for that purportedly awesome feature.
You are not alone in asking:
discussions.apple.com
How to make your Home Folder Portable in OS X
eshop.macsales.com
Keeping macOS and data on separate drives saves space on the macOS boot drive. Check the step-by-step guide on how to move your user folder.
www.imymac.com
I got a shiny new Mac Mini but couldn't afford a 1TB internal SSD to hold all my stuff, so I'm testing ways to leverage an external disk for user storage. I know I can point Photos and iTunes to
apple.stackexchange.com
