I’m still not sure this thread gave you a clear answer to your questions, did it?
(1) It’s true that you can’t have the project calculate task dates based on entering both a project start and due date, it’s only one or the other.
(2) But also, the dialog where you select whether to use the start or the due date to calculate from, is available in any project template, built-in or your own custom ones. See here for how to get to it, if it’s still unclear.