Certainly you could. If were you I would just put in a line to say "Under the terms of the entail, if the Collins line dies out, then..." just be clear to all readers that this is what the original entail was supposed to specify. Entails could be written any way you wanted to write them, so nothing else becomes relevant after that.
It is difficult, and I'm always amazed by novelists who set every book in a different time period. How on earth do they have time do that that kind of research? There's so much to know, and not until you get into the nitty gritty of a story do you begin to realize all the countless details that can be in question. But as Harvey says, that's also a part of the fun.
