It differs by company, but is not that easily gameable. (I’ve tried many times and succeeded only once in filing an application that was completely trash). The company’s patent lawyers ultimately decide what they will and won’t file, and they are more time constrained than budget constrained).

I've seen a split approach like 50% at disclosure and 50% at grant because the employer wants to see you follow through in assisting with the paperwork. Quite a few never reach grant stage or take so long that you may have even moved to a new employer.

Usually it's per grant. It's quite common for big companies like Intel to give bonuses to the employees for filling patents.

The attitude of these companies is to incentivize employees to patent whatever they come up with, whether they intend to use the idea or not.