The suggested shortcut is just a work around for the current version of Spine, which is missing visibility management function. Let me first push the discussion further by telling my thought on the questions you raised first.
How should a shortcut to toggle visibility of the attachments work?
Something like Shift+R click to hide only the attachments children of the selected bones, that works if you do it on a bone, but does not affect children bones?
Yes, if current [R-Click] is to toggle bones, [CTRL+R Click] should be consistent with that so it should only affect attachments.
I think a problem might arise when you then click again, as there would be multiple options (reactivate the first attachment for each slot under the selected bone? but what if you don't need all of them? reactivate all the attachments that were hidden in the timeline? how should this work in setup pose?) so I guess more discussion is needed on this.
As a simple shortcut like this, I do not expect it to have complex logic. The behaviour should be toggling all children attachments. Again, it should be consistent with R-Click counterpart. When use R-Click to reactivate child bones, it also doesn’t consider whether I want all of them or only some of them. It does not remember previous states for each child bones too. So I guess it make sense to do the same.
Since there could be multiple attachments in a single slot, best case is to remember and reactivate the last active one. However, reactivating the first attachment in a slot is also acceptable especially when compared to not having this shortcut at all. At worst, people who afraid this shortcut messing up their attachments could just not use this shortcut at all and do it their own way.
I think the best option on this is the dummy animation I mentioned above, I tested it and I guarantee it's a reasonably fast workflow, it just requires you to set the pieces first once.
It could be the a better option for now when compared to toggling each attachment one by one. However, isn't it only works in animating mode? and this approach is a little too workaround in my opinion. Most importantly, it doesn’t hurt to have more ways solve this problem, especially when there isn’t many attachments under each slot. Having a simple click to do the job is definitely mush convenient and faster than creating animation and copying and pasting key over. Not to mention it works even outside animation mode.
My thought is: this short-cut doesn't conflict with your dummy animation approach. In fact, they should co-exist as one of them target precise control but require more setup and the other is fast and simple best suit for simple scenario where slots don't have multiple attachments.