You are absolutely correct....and Solomon lost his Kingdom for this reason [I Kings 11:1-13].Presuming this is true, we still have direct evidence that the supposed law behind the the argument that Ruth was not a Moabite was not very well obeyed.
And Jeroboam and Rehoboan, his sons constantly kept Israel in turmoil because of the split. Israel was in rebellion against the House of David [I Kings 12:19].
Rehoboam built pagan shrines and appointed priests from all tribes instead of the Levites.....and many other evil things. It was for these reasons Israel suffered and eventually went into captivity.