Vikings get the ability to raid from two sources. Their Germanic Pagan religion and their Norse culture. The Norse culture eventually splits into Danish, Swedish, and Norwegian at which point the only thing that allows them to still raid is their religion. If you convert them and the culture has split the raids will stop. That's why it's advised to convert them, but it's actually only half of the story. The other half is waiting for the Norse culture to split up, which does tend to happen automatically, though there are a few ways to prevent it.
As for why the raids have already decreased significantly, it depends. If the Norse culture has already begun splitting and all the vikings are now Orthodox only the few that still retain Norse culture will still be able to raid. Though if the culture hasn't begun splitting yet I don't have a definite answer, only guesses.
3 actually, they are all tribal as well which also enables raiding (but not down the rivers like Norse/Germanic). Again, though, conversion helps as unreformed pagans can't adopt feudalism/found merchant republics and ai tribes tend to reform as soon as they are able.