You raise several issues here as to whether you should choose between your dad with Alzheimer's or your husband.
One, your father is 120 miles away in his home of 50 years and you feel he is lonely, yet you don't want to pull him away from his friends.
Even with your boundless love for your Mum and Dad, what do you believe your mother would say today if she learned that you feel you made a "massive mistake by not settling nearby to [your] parents when [you] left home at 19 for university?"
You mention that your dad doesn't feel you've let them down and that he wants what's best for you. This is the wish of all kind-hearted loving parents for their children"”to know that they are secure and happy.
Two, you spent all those years with your boyfriend who then became your husband to secure your future together. Are you now questioning your relationship? If you walked out on your husband, you'd have only your father. It takes time to build a new relationship. You admit that you could be left alone and childless.
At age 31, you've invested years in your current relationship and want to start a family. Yet you want to look after your dad. I get the impression that your loving heart yearns to care for another human being"”whether this is your father or your children.
Three, at some point, when your dad can no longer manage alone, you'll have to make a decision how to care for him. Why not make this decision now while he is still able to function? (You write that he still drives.)
Start by having him spend some time with you and your husband or move him closer since he gets along so well with you and your husband.
If it's only seven months, consider having your dad stay with you for a couple weeks at a time then return to his home. You could certainly drive the two or three hours each way once a month.
After a few months, other options will come to you; for example, your dad may consider selling his house and contributing to a larger home for you and your husband with private quarters for himself.
Take steps now and you will have your dad's company in this new city without him giving up his home right away. Together, you can experience the joy of his grandchildren.
I feel a lot of stress in your words"”being pulled every which way and not knowing which direction is best. I imagine, while you feel the stress of caring for your dad, your husband feels stressed about his career. Two stressed people under one roof? Aaaaaagggghhhh! Realize this and take one step at a time.
Just as you have faith while driving in the dark that the road extends beyond your headlights, have faith that venturing along this uncertain journey will uncover options you have not yet considered.