Yeah Cassani isn't really a huge improvement over Caceres... He's only more experienced in Serie A, and he's much more solid and consistent on the flank, but Martin tops him in his spirit and in his forward drives... I wouldn't mind Cassani, but renewing Caceres's loan for the new season would be the cheaper/better option for the future, as Del Neri might improve the player's tactical awareness...
Bonucci isn't really proven on a top level as you've already said, but if you think about it, we're not in the CL next year so he'll be playing in competitions that he's already proven himself in... The Europa League will be his chance to gain European experience... I don't think his transfer would be a bad one at all.