I fully understand what is in the XLT 50T package as I ordered one and probably looked at the guide 100 times LOL. It doesn't include the park assist. You are correct that it should be possible and I doubt you will find anyone here that will be able to answer your question as to why it is removed on Limited when you add that package (could be wrong!). If you could get all the parts and use Forscan to enable some things it might be possible to add it. Probably have to be some calibration as well. I imagine it would be costly to add it that way.
I have seen other oddities as well , one being when you add the tow package on XLT with 50T it will add 360 cameras but remove the built-in Nav. I mean really. It is the same question as to why they did that. I'd be willing to bet somehow it is simply to save money in some manner on their side. It wouldn't be the first time.