*In the process, I have
over-extended the head on the threaded elevating screws.

I'm not familiar with the planer, however:

Head to screws engagement:
So, there is more than one set of elevating screws? I would assume
these will have to turn in unison, for the head to lower properly.
With the head not engaged, can you see if they are turning, turning at
all, and/or turning in unison? Once you start lowering the head,
before it binds, can you reach any of the screws and manually turn
them, to get their unison-turning going properly? I would guess they
are not turning in unison and that is what is causing the binding,
maybe.

Screws to crank engagement:
Also, maybe, since the crank has reached its raising limit, i.e., in
one direction, has this limit surpassed the crank's engagement to/with
all the screws? Maybe this initial engagement, to/with the screws,
needs to be re-established.

Sonny