1/ upgrade_mod_savepoint() not used, it should be in all official 2.0 plugins
2/ $result should be replaced by exceptions
3/ incorrect use of exceptions in many places - creating new general moodle_exception is always the last option, the exception type and details should give you more hints, it is not needed to localise upgrade problems usually
4/ camel case not used in core moodle - NwikiMigrate()
5/ I guess it would be more readable if each wiki had different directory - from my pov the only fully working has to be migration from the official 1.9x wiki
6/ use DML recordsets when working with lots of data!
7/ migration from ewiki not implemented yet, right?