First, use your first instinct: SURVIVAL. Do what you must to get the best insurance you can -- if that's cobra, do that (it's more expensive than a regular ppo/hmo plan as a regular employee, but also usually better). Also, doesn't Obamacare now allow you to be under your parent's plan up to 26yo? If you're getting married soon, then get on your spouses plan if it can cover your treatments -- don't be concerned with raising premiums for the startup... this is what insurance is for.

Figure this out pronto. This is your top priority! Have someone help you!!

My wife was diagnosed with metastatic breast cancer back in 2006. I worked for Oracle at the time via the acquisition of PeopleSoft. I never wanted to stay at Oracle, but I ended up staying for 6+ years just so I can stay on Oracle's United Healthcare PPO which was the best. It covered almost everything including getting multiple opinions to crazy expensive clinical trials. I finally left Oracle in 2011 and moved over to Atlassian only once I knew for sure that switching insurance plans would not affect my wife's care, but even then, I still bought Cobra for the first month into my new job just in case.

I know it sounds unheard of for many people here on HN to stay at a job you don't like just so you can keep you and your loved ones healthy and alive, but when it comes down to it, surviving is top priority.