MA is my state and I know rather a lot about how UI works here. From both sides of the desk.

If the employer has work for you when you are medically cleared to work, it would have to be a significant reduction before you'd be approved for UI in the first place. Additionally, even assuming you were approved, the reduction would have to be 50% or more before what you'd get in UI would even match, let alone exceed, what you'd get if you took the lower paying job. And the state is able to figure that out too.

In MA, you need to be approved by the DUA to attend training school BEFORE the class starts if you're going to have a hope of collecting UI while going to school. And since your chances of being approved for UI if you don't return to work after your medical leave are fairly slim to begin with (not non-existent, but I wouldn't be betting the rent on approval if I were you), that makes your chance of approval for training hovering around, maybe 10% if the adjudicator is having a good day.