The Formsby refinisher may or may not be a better product than the mineral spirits because it is less flammable but the toxicity is higher....
As far as being in the same boat with a 50-year-old floor. If you sealed flammable liquids in that floor there may be some concerns. There is a process with any floor. I would believe the hazards to be of less concern with less age. But do not quote me, better safe than sorry, comes to mind and I am not the one who would live there.
I'll tell you my concerns are with the type, age and condition of the wood. How dry the wood may be. Condition of the sealant, the quality of its application and the time frame it was applied.
It isn't a single factor that makes your project hazardous but a number of factors put together.
I have this in my head,
A very old wood floor with the original finish from 100 years ago perhaps a very poor finish depending on which craftsman, if the person even was a craftsman, did the finish. Even today Craftsmen cut prices and use as little as possible to get the job done, I can only imagine 100 years ago. It may have even been a homeowner with little experience. For all we know on this forum your old wood floor could be very dry and ready to absorb a large amount of liquid. This floor has had a 100 years to attempt to dry out.
If the concern is for nothing then that would be great and don't get me wrong it is just that, a concern. I couldn't understand how nobody was concerned about the wood absorbing flammable liquids being as old as it is. Sometimes they assume that people doing the projects know or think about things that would seem common sense like if it seemed to be taking a lot of mineral spirits to do the job. For them, they would not be happy and may even change out the boards or change what they were using to clean them after they notice a problem. But for the people, who are asking for information, they may go through the whole project without realizing there was ever a problem to begin with.
But if you call a local professional installer and ask him/her questions concerning these issues, then you would have professional advice. For your own safety I would not except the advice from this board, others or mine as being carved in stone. In that you should realize my sincerity. If the others had real confidence in their advice they would not have hesitated to prove it.
You may even be able to have professional come out for a small fee and give you his views after seeing the wood first hand. It shouldn't cost much just for consultation.
When I bought my last car I hired a professional mechanic to check the cars out I was serious about, he charged me $75 a trip but I came home with the reassurance that I had reliable transportation. The car before it I had to shell out over $1800 for a transmission because I listened to a friend and bought the wrong car.
The only problem other than routine maintenance I have had in ten years with my car has been a recall for the replacement of the seat belt anchors.
If you consult a professional bring the results back as I am as curious, maybe more, as anyone on this forum is...