Of course it's affecting your marriage, actually from all you describe, I don't hold out much hope for your marriage. Your husband is the person in the middle and neither you or his mother seem willing to budge. You hate her, she hates you. Different beliefs just make it worse. She probably thinks your as nuts as you think she is.
Grandparents like to indulge their grandchildren with treats and things that mom and dad don't always like. (It happens....can some grandparents PLEASE weigh in on this)? I don't mean constantly giving candy, soda, etc., I just mean it happens!
Husbands become "former husbands" or as many of the "former wives" like to refer to them as "MY EX", but 99 + % of mothers NEVER become "EX Mothers".
If you love your husband and have a marriage worth saving, I would recommend trying to come to some understanding with your husband's mother. If your marriage goes SOUTH, just make a few notes now and review them when you become Mother-in-law and/or a grandparent.
Blessings.....