Which will make things really interesting as Panamanian Republicans, or as Im sure the US media will call them, terrorists (and probably rightly so) will hide in the jungles and across the borders of neighboring nations. They will probably in the very least hinder reconstruction efforts, and at most attack American settlers and companies.
As for statehood, it could actually work, if Panama would accept becoming a US commonwealth, such as the status of Puerto Rico. After a famine and civil war kills off at least one third of the population, which will leave slightly less than 2 million people in Panama. Then with massive aid from the US and being admitted as a US territory, the pro-American feelings Im sure wil be high at the time being. With US mainlanders coming to settle the country, and as you mentioned immigrant Panamanians that return home would make a significant political force. They're likely hold most of Panama's fortune and have considerable control of the Panamanian government after a short time, maybe 20 years or so.
With terrorism on the rise pressure to become a US state mounts, and the native Panamanian populace is deeply divided between continuing to be a Commonwealth territory, or to become a Republic again. A referandum is held in which the mainlanders and their off spring choose statehood win. Now you raised an interesting point, would Washington, want to admit Panama? It's likely to be the poorest US state if admitted, there is trouble with terrorists that could cause trouble on the mainland, a significant portion of the population speaks Spanish and it puts the US in a situation where illegal immigrants from Colombia and Costa Rica try to come across the border looking for a better life. There is one major advantage the US government will see and could play a major role in any decision made, admitting Panama to the Union will guarantee the Panama Canal would be Americanized permanently.
Stranger things have happened in RL, but Im not so worried about RL, just the timeline, so long as it works I won't stress.