The only incentive that works is letting such institutions crash and burn and be replaced. Even giant monopolies can end up losing money and going under.
The bare bones the market would create would depend on if people really want K-12 to be a glorified daycare or a useful tool for imparting knowledge. But having a financial incentive to function or go bust would mean you'd at least get better daycare services.
I would say yes. Most states schools provide excellent value, and I would even extend that to community colleges which provides even better value for their students.
to some sort of educator of last resort.
This might be a private school that specialized in this, or a public school that can not refuse them and is therefore full of bad students.
And how will they get to this educator of last resort? The per child transportation and tuition costs for this educator of last resort will far exceed the amount of the voucher.