Eggandbeans - if you don't mind a (long) "benefit of hindsight" comment from a step-grandchild -
My mother's father remarried (my grandmother had died) when my mother (then 18) and her older brother A (20) were already out of the house. Her younger siblings B (11) and C (8) - there's an age gap because a middle sibling died young - grew up in the house with "Kate" and eventually called her Mum. A half-sister D came along a few years later. I know my mother and Uncle A were both hurt to be excluded - even though they were adults, and had coped (and helped the younger children cope) with their mother's death, they still needed their father. And in their view, he wasn't there for them because he was tied up with Kate and the "new" family.
My siblings and I, and Uncle A's children, have always called Kate Kate. Uncle B's and Aunt C's (and Aunt D's) children call Kate Gran. We all call D Aunt D. This might have been a bit weird at family gatherings, but it was what ended up working for everyone. As a child, I'd have happily called Kate Gran, like (most of) my cousins did - but my mother clearly wanted me to call her Kate. Now the next generation - including my niece and nephews and Uncle A's grandchildren - all call Kate GG (for Great Gran).
I don't think your issue is so much what your actual children eventually end up calling Jenny - it sounds like that's years away, anyway? It's that you made a point of saying they would NOT call her any version of grandmother, when there was no need to talk about that at all, and then doubled down when she was clearly upset about it. Of course, it's your choice what your children call her - but don't risk losing half your family over a hypothetical!
You obviously care about your dad and your half-sisters. Jenny is a member of their family and therefore part of yours. It makes sense that you don't call or refer to her as your mother - you already have a mother. But children typically have more than one grandmother - so (hypothetically) why not 3? Jenny's not your blood relative, true - but I call my uncles' and aunts' spouses aunt and uncle, and they are also not related by blood.
In your place, I would reach out to Jenny and apologise for (inadvertently) hurting her. Stay away from the name discussion. Don't make any promises about what you'll do if and when you have children; just acknowledge that you understand her reaction and care that she is upset. To say she's your dad's wife to you and nothing else - I understand what you mean, but it's not nothing. You do have a long-standing personal relationship. And she's your (half)-sisters' mother, and the future grandmother of your hypothetical childrens' cousins. The "what do I call my step-grandmother?" question will work itself out in time.
As far as the holidays are concerned - I can understand being hurt about that. But you are an adult now - once things are back to normal, maybe chat with your dad and Jenny about planning a holiday together, and bringing your half-sisters (I'm assuming they're all under 18) along?