I recommend merge to m68k.
While it may be mysterious to the uninitiated, it is quite clear and common for anyone familiar with 1980s CPUs (e.g., I don't recall ever doing any real programming on anything in the m68k family, but I instantly know what it is.) Google searches for both currently show the proper Wikipedia article as the top result.
Acording to other name tags, there shouldn't be an Amiga-Shell tag at all. A shell is something that each and every OS provides in some way. For a good reason there is no Windows-Shell, Atari-ST-Shell, Atari-800-Shell, MS-DOS-Shell or Unix-Shell tag, but a tag for each of the systems/OSes plus the Shell tag.
Tags are meant to help finding stuff, that means ...
I would favor the change. However, do note that mechanical-computing is a subset of mechanical. Asking about how a daisy-wheel printer worked would be an example of mechanical, but not mechanical-computing.
I'd say no.
Tags are intended as a multi dimensional system. If we define special tag for each and every OS and each and every component thereof, the system of tags turns into a single dimensional set categories, removing any synergy in searching - beside being bloated as hell.
I've renamed 386 to 80386 by swapping the synonym then clicking “merge” – this only kept the tag wiki excerpt properly because no questions were tagged 80386. (The tag management tools, while much better than they used to be, are still a little arcane; this paragraph is documentation.)
I also created windows-3.1 as a synonym of windows31, which I renamed to ...
Yup. I'm just going to go ahead and unilaterally rename it now, since it's so easy to revert.
amigashell → amiga-shell
8 revision history entries, 6 questions and 3 "favourite tag" entries were adjusted, so the impact won't have been too big.