Hey Marc,
About olark - you shouldn't see it at all when I'm not online. And I'm not sure why it has poped up on your side, but I guess some glitch at olark.com
About your question.
1. I had customers who have migrated data from WP MU, but we don't offer any specific features to that. As far as I'm aware the only difference between WP MU and standard WP is that you have wp1_ , wp2_, wp3_ etc table prefixes. If you want to migrate all of those in the same joomla database you'll need to set the "clean migration option" to no. And then run the migration for the first prefix. Then run the parsing. Once ready change the prefix to the wp2_ do the migration (since clean migration is set to no, we will add the new data to the old one and we won't delete any posts that we already imported) and you go on like this.
2. We don't do anything with the wp_ tables. You have to manually delete them.
Kind regards,
Daniel