Re: is Dale winnable?
It's all about luck .. 
As you saw by yourself dealing with Ruhn is not impossible, so if you go for short campaing it is surely winnable, but in general you have to be lucky and be able to build up your empire and create a huge economy (thanks to Ruhn cities) to deal later on with Mordor and/or Harad.
As far as I saw as long as Gondor will keep at bay Mordor and Harad any "good" campaign is winnable, but if they will be wiped out relatively soon, one of this two factions (usually Harad) will become really too strong, and you might have a small chance only if your economy is really strong and allows you to maintain some extra stacks. In my experience I only had to give up a "good" campaign when such an event (Gondor completely overrun) happened.
Anyway, in this case, you can try to retreat towards your original territories, turtle and defend Ruhn's ones, build up economy there and hope to have enough time to be able to field some new stacks...
Ciao
Flinn