I've used the same hack, with the same results - my answer was to keep it in the Sims inventory until it was actually needed, then put it away again after they've used it.

The question did not ask how, only if it was possible to learn witchery from other playables.  Your answer did not address the question accurately, as the OP wishes to know if a playable can learn from other playables, not be forced into becoming a witch by another playable casting a spell on them.

Yes.

5) Does all the Seasons garden stuff work on community lots?  Looks like it does to me, but then my apartments are possibly borked...
I built a big communal gardens at the back of one terrace including orchards and a greenhouse.  The playables could use communal compost bins and plant things in the communal garden and the landlord was sweeping leaves, weeding, watering and pruning. Is that because of an awesome hack or basic EAxis? I didn't get as far as harvest, but it was all working rather nicely!  Does the garden club look at the communal area or just the appartment?

The Garden Club can't be called from an apartment lot, but community gardens do work rather nicely, as the landlord will water/weed for you when appropriate.  This is basic Eaxis, no hack needed.