[Mittelwaechter]

100 meters? What do you expect them to do at this height? They don't have torpedos to attack but bombs to drop from above.

Have a few wings of three or four bombers attacking from different vectors at 900 meters at two thirds of their top speed. They'll try their best to damage some freighters but don't expect too many of the flyboys to survive.

Two of them will sink a lonely destroyer easily, but a flock of armed freighters is something different.

Give 'em the heavy loadout and make 'em experts - and the freighter crews should be novices - and you may watch a dramatic and longer lasting circus.

EDIT: just opened the MIS of this single player mission to get an overview.
Well - the script is kind of simple, true. To change the skill of freighters or air support you would have to modify the campaign MIS itself.

But you could shadow the convoy and radio every hour for air support - after your 1WO has discovered at least three contacts.
There are German bases in Norway and the game will send help - if in striking range of the aircrafts.
But only with a certain probability - to model different weather conditions or lack of fuel or ordnance.

Your job is to sink 15000 tons - no problem I guess.
And as a rule of thumb: don't rely on the Luftwaffe, Herr Kaleun.