Is there any consensus as to the internal organs/stuff which maybe doesn't fossilize well? Like, did they just evolve a bitchin' chassis but they're constantly tinkering with the internal bits?
In truth, the modern horsehoe crab is different than its ancestors, just not as much as usually occurs in several hundred million years. So Frank was really more like, "Eh, maybe a little, but not much."
Fossilisation has a bit more to do with deposition conditions than the creature itself. It's why we know next to nothing about jungle dinosaurs and ones around mountains. Most of our knowledge comes from marine and wetland dinos.
They would most likely look the same but there are still evolutionary changes deeper in the DNA that aren't visible like metabolic and digestive functions. A modern horseshoe crab would not be able to reproduce with one from a 100 million years ago despite looking virtually identical.
Triops: tiny freshwater horseshoe crab looking guys. You can buy their eggs for cheap online and raise and breed your own. Easier and cheaper than SeaMonkeys.