Hi, Clavicle. Testing for shellac is easy. In an out-of-the-way corner, put a couple of drops of denatured alcohol on the finish. This will immediately eat into the finish if it is shellac. There is no similar test for the difference between regular varnish and polyurethane varnish. Since there was no water based poly 30 years ago, you know that it was not that. I think that you might have some lack-of-adhesion problems between water based poly and your floor. Again, do a test. Put some of the water based poly on a small test patch and let it dry. Try to scrape it off with your fingernail. If it shows ANY signs of flaking, don't use the water based poly. That little bit of abrasion will be nothing like the abrasion that the floor will get over another 30 years.
For what it's worth, if it were mine, I would do a complete sand and refinish job.