Is there any historical agreement as to the spontaneity of Parks' protest?

There isn't. Not to say people really disagree or quarrel over it, but to my knowledge there hasn't been any evidence presented how exactly how much planning went into the event.

Now it's definitely true that there is more to the Rosa Parks story than is usually presented. To begin with there had been news of Black women being kicked off buses and treated poorly for a few years before, and talk of a boycott had been around too. Parks was actively involved in the Civil Rights Movement before the bus incident. She was college educated, had attended the Highland Folk School, and served as a secretary in the NAACP. Its not clear whether Parks specifically rode the bus that day to get arrested, or whether she was riding the bus and saw an opportunity and took it. During that time the NAACP was actively looking for test cases to challenge Alabama's segregation ordnance. Parks also knew one of the lawyers, Clifford Durr challenging the segregation. A few women before Parks had been arrested for bus-related things. Ultimately though it wasn't Parks case that went to the Supreme Court, it was another case involving the other women who had been arrested before Parks.