In addition, is there any truth to the anecdotal claim that many Koreans transferred fairly smoothly from senior positions in the colonial administration to senior positions in post independence South Korea?
The extent of collaboration depends on the social status and the time period. The wealthier the people, the more likely they were to collaborate. That said, Koreans were generally quite poor so there were not a lot of collaborators. Also, pre-annexation, "collaborators" refers mainly to the people in the Korean government who pushed for integration with Japan, and to the actual government people who signed the Eulsa Treaty. After annexation and the integration of Korea into the Japanese empire, the issue becomes much blurrier. What does "collaborator" mean? If we're talking about people who turned in rebellious Koreans who were fighting Japanese imperialism, the number is quite low. If we're talking about people who didn't put up a fight when the Japanese came in, the number gets bigger. If we're talking about people who accepted the Japanese presence and did their best to live their lives, the number is even larger.
For consequences of collaboration, there were some investigations and prosecutions under Syngman Rhee from 1948 until 1950, but they handled a small number of cases (under 1,000). I don't have a lot of information on it. However, under President Roh Moo-hyun (2003-2008), the government started prosecuting collaboration cases again. Since that falls in the 20 year time period, I'll just put some links to news articles about it and you can see what has happened.
About the transfer of power, yes, it is true. When the US military took over in late '45, the Japanese administration was left in place - the US had no meaningful experience with or knowledge of Korea. This was not the most popular of moves and changes would come under the Rhee administration, but the fact is that the Japanese government was a strong and efficient one. Many people who worked for the government under the Japanese ended up working for the new ROK government.
Of course, the most famous collaborator is Park Chung-hee, the general who took power in the 1961 coup. He attended the Japanese run Changchun Military Academy in Manchuria and went for further study at the Imperial Japanese Army Academy in Japan. After the war ended, he went to the Korean Military Academy and went into the Korean Army afterwards, the same path that most collaborators would have taken.
Take a look at Populist collaborators:the Ilchinhoe and the Japanese colonization of Korea, 1896-1910 for some information on pre-annexation collaboration.
Here are some of the news articles on the more recent collaborator cases:
Politics, price of Seoul's collaboration probe
Committee okay's seizure of Collaborator's property