1. Avi K, Abarbanel is not a Rishon (heard in a shiyur). Even if he were, he is not of the same standing as Rambam and Ramban. And yes — a Jewish king cannot just do what he wants. He could be tried by the Sanhedrin, and my need their confirmation to start his reign.
2. It is an accepted rule in the world of Achronim that one can not argue with a Rishon unless one has a different Rishon to back him up. And your statements are outrageous: according to you an Achron is allowed to argue with Rishonim, but a learnt person nowdays can not say that an opinion of a recent or a very recent Achron is shvere.
3. When Ribono shel olam gave the mitsva to appoint a king, he realized that some of them might be bad, won’t you argee, Avi K?