What are the benefits of a two-party system versus a multi-party system based on coalitions?
There are no particular benefits, in my view. If I were to design an election system for America from scratch I would probably ditch the presidential system altogether and use some sort of PR-plus-state-reps system. Basically have the House and Senate together elect a President or First Citizen or whatever and have them run the country.
Could the Democrats or Republicans reasonably decentralize into multiple collaborating parties?
In practice the Dems and GOP are already basically two large coalition parties. The internal parties are just informal, although there are subgroups like the Congressional Black Caucus which are basically representative of a particular aspect of the coalition. I'm not sure what formal coalition party structure would add to the conversation.
edit: FUK U OHIO