Your example had the $12 job giving you $16, and the $15 job giving you $16

What’s the difference?

What stops you doing a deal where you (on paper) work 80 hours a week for $8 rather than 40 for $16, you’d cost the employer the same, but you would get twice the income ($1280 a week).

How do you even define when you’re working.

You'd make more money taking the $15 job than the $12 job. Your W-2 says how many hours you worked (and how much overtime, if any). The federal government has access to those W-2s, and can determine the subsidy amount using it.

Your example said

12 plus 4

15 plus 1

I’d expect it to be more like

12 plus 4

16 plus 3

20 plus 2

24 plus 1

A 20% marginal tax rate.

The hour thing seems massively corruptible

The wage subsidy would only close the gap between the wage and 16. If you made 16, there would be no subsidy.

Earn 16 get no subsidy

Earn 15 get 1 subsidy

Earn 12 get 4 subsidy

What’s the difference to the worker?

Fork 4/3rds harder to earn 16 not 12, see no gain. Equivelent of 100% marginal tax.