Still working on the pictures of this one. Stage IV: 5 years ago I did the very first part (HochweiĆsteinhaus - Ćfnerjoch) and the second half from Giramondopass to Untere Valentinalm on three different days. Part of this has been described here:
You might also want to look at stage 2 and 3 by Vid. The whole trek is 180km long (the ridge is 140km but there are some detours). April might be too early, especiallly in stage 3 which is not sooo easy...
Bivaks - yes can do that but there also are a lot of huts which have winter rooms.