Are you reading The Religion of Java now? :)
In this context, "Abangan" is originally from a Javanese term that means "red". Meanwhile the other "Abang" that refer to a young guy is from Betawi language (people of Batavia). So I don't think that they are both connected.
As far as I know "Santri" is a term used for people who learn about Islam (Either it's modernized or traditional) in Pesantren (Pesantren is like boarding school for people who learn about Islamic way).
Priyayi is a term used for highborn, and yes they work with the Dutch.
As for Abangan and Santri, you can differentiate them from their behaviors and their way of life. Santri are the one who follow the Islamic way, and Abangan are the one who doesn't really care about living in Islamic way.
As for Priyayi, it actually wasn't defined by their faith but more to their blood line. Most of the Priyayi learn the Islam that mixed with traditional Javanese beliefs, but some of them also learn about modernized Islam.
The same thing also applies to Santri, some of them learn the modernized, and the other learn the one that mixed with traditional Javanese beliefs.
Another term for Muslim leaders are Ustad, Kiai (be careful, although lot of people write it with "Kyai" the correct pronunciation is using 'I' not 'Y'), Ulama, Ajengan and Buya. (Though most peple usually use Ustad, Kiai, and Ulama).
Hope this is help! :)