I've got my left hand on the mouse, my right hand on the arrows... am I supposed to use my dingdong to click the number keys? I have eyesight issues and coordination issues (I am 60 something) and usually go for turn-based, so what may be obvious to you is quite foreign to me.

Alternatively, is there a way to play the game successfully with a mouse-click-only build?

I don't believe there is any way to play mouse-click only, but offhand there are two options I can think of.

The first is the WADS controls, which at least puts 1-5 in relatively easy reach. The second, of you prefer the arrow keys, is to maybe try rebinding the quick-slots to the numeric keypad, if they don't work already.

Alternatively, the XBox 360 version of Divinity II (If I recall correctly, haven't played that version in years) had 8 quick-slots that were bound to the plus-pad and the A, B, X, and Y buttons on the controller. Perhaps for something like this, a controller would work better for you? If the game doesn't have controller support, there's always the program Joy2Key that you can use to bind the controller to keyboard buttons.

Anyway, Hope this helps!