Lalli serves as scout. Tuuri serves as driver,, skald, and mechanic.
Considering these jobs in reverse:
No one expected them to need a mechanic. The plan was for them to explore near the tunnel for a couple of weeks, then return. In case of any disaster, the survivors would be expected to flee back to base, in the tank if possible or on foot if they had to. And, in fact, the tank did hold up without needing a mechanic's care for weeks until it was damaged by a troll. So a mechanic wasn't needed and that wasn't the job.
I don't think Onni could have been hired as a skald. His Icelandic is "special" and he doesn't seem to speak or understand Swedish, so I doubt he can read Danish. Mikkel, however, can. On page 270, Mikkel looks at the medical journal and says, "I'll definitely need to take a closer look at this one." Tuuri says, "But that's my job!" This makes me suspect that Mikkel was supposed to be the skald as well as the medic, and that Tuuri's academic skills were a bonus.
We don't know if Onni could serve as driver. It may seem sort of unlikely that he could, but it's not like Tuuri was a very good driver. Maybe they both were allowed to try driving a tractor a few times in Keuruu. Alternatively, maybe the sponsors expected one of the others to figure out how to drive ("How hard can it be?") So maybe Onni could have been offered a job as the driver, but not necessarily.
Finally, Lalli is the scout. They absolutely need a scout, and no one else appears to be qualified to be one -- which means that that is the job Taru offered to Onni. But how could Taru expect Onni to be a scout?
My head-canon is that Onni is able to send his spirit out to scout while his body remains safe behind walls, and that's the job Onni was supposed to do on the team, and what he did do in Keuruu.