Very interesting! Starfleet's motivations for supporting a similar system would be different, of course, but perhaps no less practical or valid.
Odds and ends:
Can remember off hand whether Spock is specifically stated to have left Starfleet, however he was "On Vulcan, apparently to stay" and had no intention of returning.
To Kirk's knowledge, yes. But Spock might see no obstacle to returning to active duty once he got rid of those pesky emotions for good, whereas humans might take the going-to-monastery thing the wrong way due to cultural bias...
There's also no suggestion he ever resigned, it was a leave of absence. He was allowed to keep a personal Runabout with him after all.
Indeed. The only thing that makes the leave of absence unusual is that Sisko says he doesn't know how long he will be gone. But that may simply mean "I don't know if my quest will take all of my allotted leave, or be completed in a few days already".
It's a bit funny that Starfleet allows its personnel to use runabouts for civilian errands, but there's plenty of precedent. The O'Briens did that in "Tribunal" for one. And Sisko was on a holiday trip in "Jem'Hadar". And never mind the times Sisko used the runabouts or the Defiant
for missions of his own making (such as sorting out Dax' past in "Equilibrium").