I'd say there are a lot more factors than the ones you listed, cat, but just wanted to point out that "Covers screen space" should probably more accurately be divided into "Uses up screen space that could be used otherwise" (eg. verbcoins in earlier LucasArts adventures, inventory in first Kyrandia, I think, etc.) and "Covers the playable screen" (eg. verbcoins, Sierra's VGA games, partially).
And technically, the number of interactions in all of them (except the last two) could be as many as the dev chooses (or can fit on screen)

Also, I'd say a more scientific use of ergonomic might be better. I think I remember the last thread about verb coins brought up
Fitts's Law, which is quite useful here. So something like the LucasArts or Sierra verblist, with the buttons along the edges of the screen is actually MORE ergonomic, because the size is infinitely wide (in the case of Sierra; In LucasArts verblist, the lower buttons are infinitely wide, but the buttons themselves are quite large as well, so I'd say they are still ergonomic- and all this is talking about mouse, not touchscreen or mobile). Verbcoins, however, tend to be fairly small (otherwise they cover up too much of the screen, which is bad), so that can make them unergonomic.
Judging from my preliminary investigations, at least in terms of Fitt's law, verbcoin vs Sierra/LucasArts could be almost equal, but if we're talking about the hypothetical "perfect" solution, well done verblists could probably still win out (again, the limitation that verbcoins can't be too large comes in).
I'm trying to find a game screenshot to be able to approximate the values, but I'm tired and this seems complicated (the way players would use the two systems aren't really the same, so I'm being sure how to make them comparable). Someone else do the hard work? You might prove that verbcoin is even better (at least insofar as Fitts's Law goes)

!
As a side note, talking about Fitts's Law, I started wondering why LucasArts didn't put the more common interactions (I'd say maybe Use/Talk/Look, or something other than Look if people figured out the right-click shortcut) right at the bottom...
PS: I just tried BaSS right now because I didn't really remember it, and I think it gives a very interesting solution for the "People don't even realise they can click the other mouse button" problem. With the right click to interact, left click to walk/look, as a casual player, you would begin by left-clicking stuff, which gives very obvious "Look" responses, but there's no way to continue without interacting with stuff, so you'd end up right-clicking (people might not think to click and hold the mouse, but they would eventually right-click, I think).