What you could do, and this is a bit radical so bear with me, is play the dark elves completly differently.
Since their campagin is a bit broken, abandon it. Instead of trying to cling onto your cities, just leave them and migrate somewhere else. You really shouldnt be too attached to them because lore wise the DE are in the region on a massive raid, so their not too concerned with holding onto some pretty bleak settlements in norseca.
Instead, make as many units as you can before the HE rape you, sell ALL your buildings in all your settlements, make a couple of ships, and send EVERYTHING you have to marienburg.
BEST. RAID. EVER.
Capture the city and start from scratch. After 20 or so turns the money will pump into your treasury and theres plenty of room to expand into the rebel and beastman territory in the surrounding regions. Eventually you'll be strong enough to make a serious expansion and you have plenty of options, Nulin, Aldorf and Middenheim (3 of the richest cities in the empire) are within resonable striking distance, and if you manage to control all of them then you pretty much have infinite money
I think the idea of desperatly trying to defend a couple of settlements in Norseca is pretty pointless as it goes against the lore, the DE like to raid weak, defenceless targets, so let the empire feel your wrath!

The HE can have Norseca, the empires where the loots at!
