I feel for you that you have to go through this at a time when your own kids are so young and need so much care. I lost my mom to stage 4 lung cancer about 3 years ago and I didn't have any children at the time and I know what you are facing.
I am assuming that your mom is not 65 or older or she would be on Medicare and that she does not have a spouse whose plan she could be on. And if she retired in the summer it would be too late for her to get COBRA coverage from her previous employer.
At this point it is going to be very hard to get insurance outside of Medicaid (aka Medical Assistance/ Access/ Access Plus) because she has a pre-existing condition. I would really encourage her to go on Medical Assistance because I know my mom's medical claims totaled over $600,000 and that does not count the prescriptions or what Medicare paid toward hospice. None of the treatments that my mom received were all that exotic either, but everything adds up. BTW, a lot of the drug companies offer assistance so if they put your mom on any prescription medications find out who the manufacturer is and call and ask about their assistance programs.
I was working for Highmark at the time my mom was ill and she had our coverage, but her drug coverage was limited to $350 a quarter and she was on some prescriptions that cost thousands of dollars but she was able to get them cheaper through the drug company's assistance plan.
I also cannot stress enough that you will need support through this process because things can change so quickly with this disease. If you are working, I would encourage you to apply for FMLA as soon as possible so that you are not trying to deal with this right at the time that you need to be off from work.
Have a frank talk with your mom, pride may not be the only thing holding her back from going on Medical Assistance. Medical Assistance requires you to spend down on assets that you have and your mom may feel that if she does that then she won't be leaving you with anything when she passes away. Parents can get very funny about those things when they are facing their mortality.
I wish you all the best.
PS
You can also ask your mom's doctor about clinical trials because you generally do not pay for any treatment that you receive in the clinical trial