You're exactly right in your description of how this all works.
You're also right that there's not a "reset" option in the Master view.
The "reset," as you know, resets a slide to its master's specifications. But
it's not available for the slide masters, because, I guess, there's no
master to reset the master to.
I would guess this was an oversight by the development team, but it may have
been "by design" -- that is, for a reason. One workaround, though, is to
insert a new slide master and move (drag) the layouts up into the layout
list for the original master. Alternatively, insert a new master, then use
the format painter to apply formatting quickly -- to various objects and to
the slide in general. (You can format paint from one slide master thumbnail
to another to get some of the basics, then you have to do it to specific
objects to get the specifics.)
I know this isn't ideal. I'll report your "missing reset button in slide
master view" issue, because I definitely agree this should be available.