Sorry my mistake, I guess I read incorrectly what you wrote. To answer your question I'll try my best here:
Insurance I'm not sure what state you live in but when I was working full time making around $24k I still was able to get Medicaid. It depends on your state. Some states have application processes to raise the limit from $25k to $36k/yr income if you have select illnesses (CF being one of them in PA at least)
If you are past the 29 month waiting period (sounds like you just passed it) for Medicare Disability insurance it should not effect you... you'd have to check with them I'm not well versed in Medicare.
But essentially Medicare is the SSA level and Medicaid is the State level, most states its around $24k so you shouldn't have a problem.
The reason you are coming out with 240 a week over a year being over $12,000 is because when you calculate monthly you have to multiply by 4.33 not 4. There are 4.33 weeks in a month. Which yes would but you over as far as the payments to you is concerned.
However keep in mind when you start out working a job while on SSDI you can make ANY amount of money without having your payments change for the first 9 months. After that they will knock your payments down if you are making more.. basically subtracting anything you make over $1000 from your SSDI payment.