Cresti, don't torture yourself...
Just accept that you cannot get easily from Kargil to Darcha, take a look at Google Maps. Only way would be through Leh. From Darcha to Spiti Valley it takes 2 days, partly on scenic and good, partly on scenic and terrible roads. I stayed one night at the turn-off from the Leh-Manali-road to Spiti valley, it was not too exciting, a few huts, restaurants, tyre repair service. When the bus came next morning, it took a full day to get to... I even forgot the name of the village, but it was somewhere in Spiti valley, with all the backpacker hostels etc.
This is the Himalaya, and it takes ages to get from one place to another. You've been there before, so you know it... No high-speed trains, no motorways...
Of the 3-week Zanskar trek the better part is undoubly the first half, to Padum, starting in Lamayuru (or doing it the other way round). There are a few possibilities to avoid part of the road that has been built. I promised to email this to another person, if you're interested, pm me your email address, then I can send it also to you.
And, yes, you will be there in the main tourist season, if you want to go on your own, doing mainly homestays and camping, no problem, but if you want to look for a guide with pony to carry the luggage, you might have to count on 1 - 3 additional days, because there might be no-one available just when you get there (Lamayuru or Padum, for example)
As I wrote in the post before, try - no matter how - to get more time. 2 weeks are next to nothing up there for doing some real trekking...