You may want reduce the amount of OOUI overrides to prevent future regressions (and provide better skin support).
I would recommend having the pin and collapse buttons as simple icon-only buttons (right-aligned, instead of absolutely positioned), instead of the current nested buttons.
The ability to expand the toolbar by clicking on the label (or the space either side of it) can be re-added using a click handler to trigger the expand/collapse button, a bit how field labels can trigger checkboxes.
The "root" of the problem seems to be that the frameless buttons can not be toggled. If there is no particular reason against this it seems to be the most sustainable to add toggle mode to the frameless button class (or however OOUI calls "class"/"template"…). If that is not possible for us, I would leave it as it is.
It might be more of a PM decision, actually: Change it now to the standard and reduce maintanance while paying with a less good UX or vice versa?
Just to repeat my take from the original task, I'm against putting an interaction element with visual appearance of a framed button there.
The current treatment/look of the items is good IMHO and should stay the same, even with a different technical implementation. The technical concern on nested buttons by @Esanders remains.