(a) there is no downside to your plan to leave the dunkel at a higher temp for a few extra weeks.
(b) you should use your hydrometer, not your calendar, to decide when it is time for a d-rest. In other words, 1.020, not "two weeks."
(c) pitching onto the entire yeast cake may not be best. In this case the downside is low because your yeast requirements for the doppelbock in terms of cell count are pretty substantial. But in general, take the time to pitch the correct amount of slurry, which is often anywhere from half a cup to two cups. There is a ginormous thread somewhere on overpitching with an entire yeast cake. Slurry is the absolute best source of fresh, viable yeast but there is such a thing as too much. Get your slurry out into a sanitized container, clean your carboy well and re-sanitize, then pitch however much slurry you need to get the right cell count.
(d) What yeast strain are you using? I'd try to go lower than mid-50's if it's feasible.
2012 Canadian Brewer of the Year
2013 Canadian Brewer of the Year
2015 Canadian Brewer of the Year