You may be working, but say you are still in a financial hole. Time to go to the welfare department to make sure you have full health ins for your son (Medicaid) and ask about getting some 'day care' help which is usually in the form of a voucher, which would cover your son staying until you picked him up.
I am concerned for you and your son, as although he may not understand everything his grandmother is saying about his father he does feel the tension.
As you stay around her and read and continue to post, you will come to understand when we say NO is a complete sentence. Now I know that is hard to say to a parent or sibling, but sometimes we have to, we have to remove the toxicity from our lives and many times our families are toxic. I had to go no contact with my own mother for 5 years. It is not easy, it was not easy, but for my own sanity it was necessary.
Your mom has her opinions about the whole thing as it is now unveiling and of course she wants you not to hurt. However, her opinions do not have to be foisted on you every time you talk or see each other. It does become easier when the subject changes to your AB to say 'sorry I have to go will talk to you later.' If on the phone hang up, if in person, look at your watch, say the above, grab your son and split.
Eventually it will start to sink in that you leave or cut the conversation short when she gets on her soapbox. I know you love your mother and I know you love your son, and this whole mess is just that a mess! Small baby steps, it will become clearer.
Love and hugs,