Secret agent is how I've done him in fics, with the 'constantly being fired' as his excuse for having to suddenly be somewhere else with no warning, when he finishes a job or is reassigned. Read the character profiles of Trond and Taru, way back in the early days of the comic, and you'll see why I figured they were running Mikkel. 'Pursue something more interesting', forsooth. Several of the other writers have done similar with him. He says he isn't a doctor, so my bet is on some kind of behind-the-scenes researcher, possibly blackmailed into the trip, possibly there by his own choice.
We'll see how Minna writes him.
And as for Tuuri, might be that by the time the tank is repaired and they have tried whatever they try on Tuuri herself, they arrive at the rendezvous point just in time to see the ship hull-down on the horizon, having been called back, or avoiding a leviathan, or refusing to take aboard a potential troll, or whatever.