1WO means...

[frau kaleun]

FWIW I doublechecked Mulligan's Neither Sharks Nor Wolves which is the most recent thing I've read that gives a fairly thorough overview of the "typical" crew makeup. He states that the 2WO was usually a Leutnant and adds that

By 1941, U-boat crews often included a midshipman (Fähnrich zur See) or two to understudy the watch officers of the LI... If a senior midshipman (Oberfähnrich), he might supervise the third bridge watch and assume an officer's duties. Regular midshipmen, however, did not enjoy officer status on board, ranking instead as the highest-paid noncommissioned officers (NOCs).

That's pretty much in line with everything else I've come across on the subject, which admittedly is far from an exhaustive study... I still think, though, that having even an Oberfähnrich as a First Watch Officer would be quite a stretch. That would make a mere midshipman the second ranking line officer on board which seems highly unlikely, especially since it's my understanding that someone at that level would not have been through the standard "commander training" courses yet.

Of course I could be way off on all of this, lol.