The American plan was a last resort plan to guarantee American access to Brazilian rubber. It was also a negotiation tactic to let Brazilians know that they Americans /Allies expected complete access to Brazilian rubber and would not tolerate Axis friendly country in the continent.
The only description I have read on Operation RUBBER, that draws from direct research into the original documents was in the Naval Institute Proceedings over a decade ago. The author described the plan as aimed at seizing the series of all weather airfields spotted along Brazils northern coast. These airfields were used by the Allies for the trans African/African air route. There were also, on paper at least in transport range of VLR Axis cargo aircraft & in theory vulnerable to Axis airborne offensive from Dakar & other west Africa airfields in Axis hands. The author found this intelligence analysis of the north Brazilian vulnerability came from the Office of Naval Intelligence, and suggests they were a bit overwrought in their findings. In any case the fact is King as the recent appointed Chief of Naval Operations approved a rehearsal of Plan RUBBER. This was executed by Amphibious Forces Atlantic Fleet in March 1942. The 1st Marine Division & 9th Inf Div were landed on the North Carolina Coast. That area resembled the target area of Brazil. That is a long chain of sandy coastal islands fronting a interior of linked bays, estuaries, coastal marshes, and alternating forrest and farms further inland. There was nothing in this article that described Plan RUBBER as aimed at the urban centers of southern Brazil.
Through early 1942 the air route was serviced by civilian contractors, mostly US airline companies and Brazilian subcontractors. During 1942 the US negotiated the ability to station unarmed US military personnel at the airfield as part of the service establishment. Exactly how many military personnel were sent is not clear from the Proceedings article I read.
A look at the old maps of Brazil from the 1950s & 60s show the north coast as relatively isolated from the densely populated southern region. The maps show no interior population or infrastructure of note, and no overland or trans Amazon River connection with the population centers of the south. The coastal population of northern Brazil depended on sea & air transport for communication with the rest of the nation.