The league has a near impossible task now of finding 32 starting caliber QBs, so limiting games a player can participate in for an expanded season would put a preseason-esque product on the field. Though I doubt the NFL really cares about that.
I think Kizer is getting the extended look as the #2 because he shows it in practice where the play speed is significantly decreased. In the games last year and this preseason he just doesn't look comfortable with what's happening. Better than Hundley, and a lot like Seneca Wallace.
Boyle still looks raw, but he does not have happy feet and works the progressions. He just has some accuracy issues which could be attributed to rushing the throw when he sees the receiver coming free (inexperience). Hell, Rodgers didn't look all that comfortable is first preseason. (not comparing Boyle to Rodgers). I would go with him as the #2 unless a surprise cut allows for a better option/upgrade.