There is no correlation between "full page navigation" and "not passing users' passwords around". Full page navigation is only needed for on-premises auth servers that require a redirect, and the server can easily tell which users need it as soon as the username is typed in. Give them the redirect they want. Everyone else gets a better UX.
Ah, yes, figure out which ux the user needs before they tell you who they are. Please, let me know if your genie is for rent. Otherwise, see other comments for how the detection ux with a default password entry caused issues for us.
