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So, can you do the switch mid-line, and create a display with text and hires side-by-side (at the price of making the programming restrictions even more horrible), or is the switch so slow that that introduces horrible display artifacts?

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You can. Although I have long since lost many of the details, I used the Apple II to present subliminal perception data in experiments for a cognitive psychology lab that was an early programming job for me in the 80's. We had to run an oscilloscope on the signal to catch the vblank to time our display onset properly.

Try watching the demo using that technique called the "Apple II 64k Cycle-counting Megademo" at https://www.youtube.com/watch?v=McKAcKVTIQs . There are not any artifacts with the changes between text and graphics mode in the middle of lines.

there are no artifacts in the recording because it was recorded on an Apple IIe which has slightly better video hardware. You do get some glithces on the earlier Apple II+

Later models you can bypass the floating bus/vapor lock and get VBLANK from a register (but IIe/IIc/IIgs all do it differently). I coincidentally released a 256 byte demoscene demo using this today: https://www.youtube.com/watch?v=iWq57yeu4gQ

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