[goldsac]

This confirms that the button names of <rewind> and <fastforward> are indeed correct, so it seems that something else is blocking their re-assignment during fullscreenvideo mode. This also causes my optimism to wain, as I can't really think of anything else to try unless anyone else has further perspective on the matter. It's not the biggest deal, as it's pretty quick to activate the OSD and modify the subtitle and audio settings manually when needed - but I'm a tenacious little monkey, and wouldn't mind knowing the answer to this for its own sake (as well, of course,  as having 100% of the settings I want).

You may already know this, but next audio/subtitle is by default mapped to longpress-up/down.

I don't know why it's not possible to remap fastforward/reverse. I tried it myself (using the keymap editor), and couldn't get it to work.

Edit: there's a bug ticket for this here: https://trac.kodi.tv/ticket/17979

Apparently it's intentional. :-( 

Oh cool - I didn't know about those default longpress assignments. Is there a source for those, or did you just happen to discover them on your own? 

And thanks for the bug ticket, as that's the kind of definitive information I was hoping to get. Looks like it's going to be a difficult one to overcome, so I'll just have to go forward without those customizations.