OP, I get that step families are hard - trust me, I know - but what went on when you were a teenager, any perceived slights etc, need to be re-reviewed with your adult head on.
As someone who had step parents on both sides as a step child I know it's very hard being accepted into a 'new' family by someone who isn't your parent, and tensions and frictions are always going to happen, especially when other kids get involved. I butted heads against my step mum and step dad all the time as a teenager but as an adult, in my 30s, I can look back and appreciate it was difficult for everyone at the time, and as adults we've moved on and now all get on.
Having said that, your dad has been with this woman for a long time, and it sounds like she's done her best to include you. I can imagine why she's hurt you're insisting that any potential children you may have call her by her name, and not by a grandparent-esque title. Ultimately it's your choice, but would it be that bad if you compromise and use a variation of names between the grandparents/step grandparents?
You could have granny, grandma, gran, nana, grandpa, grandad, gramps etc etc. I 'named' my four parents out of a combination of these and everyone was happy because I didn't differentiate between my biological parents and step parents; everyone in my child's eyes is a grandparent, and I suspect your step mum is upset that you've singled her out and don't consider her as family. In this case, you can't blame your dad for supporting his wife. If there are further, deep seated issues with you dad and step mum then that's something you'll have to deal with, but in the meantime I suspect an apology is due.