What I'd do then, is to map an extra pair of Up/Down to the mouse wheel Extend keys (W/R) as they do a bit of the same when scrolling. I think I did that for my TMK files.
For the mouse movement keys, maybe Ctrl+Left/Right/Up/Down for now? Those can be handy and this way reduces chording a bit (at the cost of stretching). They'll be remapped when the proper mouse keys are back up, so it's no biggie anyway. But that's a good placeholder I think.
Not sure what to do with the buttons 1/2/3 then. Button 1 can be Space for now, as normal Space is missing in the Extend layer and it can be handy in some dialogues. Button 2 can be an extra Menu key as that is often its function. Then I'd make Button 3 into Ctrl+T as mentioned, as a middle click will sometimes make a new tab.
Did you lose Esc on Q? I find that very convenient indeed, not only for Vim but for dialogs too! Any other deviations?