If the Magus has 1 level of Witch, their Familiar is a Witch Familiar who can learn spells from other Witch Familiars. Of course, if they only have 1 Witch level, they couldn´t cast these Witch spells beyond 1st spell level.
But the Knowledge Pool and off-list spell scribing to Spellbook made me think, why not allow Witch1/MagusX to prepare Witch spells their Familiar knows in Magus slots, possibly at a Spell Level Adjustment Penalty? The advantage of this over the crazyness of Knowledge Pool is that it´s still subject to the normal limits that exist for Witch Familiars learning Spells, e.g. meeting other Witches and paying them/trading (which a Witch1/Magus X wouldn´t be well positioned to do).
Thoughts?
Yeah, it would be a pretty easy switch-out...
Basically just modify Spell Blending and Greater Pool Spell to say Witch spells instead of Wizard spells. To get rid of the need for 1 level of Witch (which is silly), the Spellbook could be replaced with a Witch Familiar from the get go (given it would be a class variant, e.g. ¨Witchus¨ 8-p).
Of course, if it were to be full-blown variant (I don´t see much room for PrC if you still want to use Grt. Pool Spell @Magus19), there should be some other Arcana and replacements/variants of other core Magus abilities, probably including spell list as well.
The ability you suggested in your first post is actually quite powerfull, since the witch has access to some spells other arcane casters just don´t get. I would require at least the Broad Study arcana for this to work, and even then with a cost like 1 pool point per prepaired spell.
The second suggestion to reflavor the Magus with a Witch component would require to change a lot about the spell list, more towards debuffing and a lot less effetive damage spells.
If you were willing to make those changes you could add an arcana that allow the stacking of magus and witch levels for the purpose of save DC calculation and hex effect.
THat all sounds about right... Some of the details depend on whether this is a PrC depending on having 1+ level in Witch or a ´variant´ Class to begin with (the 2nd really works better). Especially in a ´variant´, it`s very easy to not only change the Magus spell list, but remove references to Wizard spells, so the Witchus would not be able to gain access to Wizard spells at all, balancing access to different Witch spells.