Agreed contrails are a very complex subject and their formation and dispersal depends on a range of variables.
I've seen them in winter formed briefly as low as 9,000 feet in very cold, saturated, air.
As to the bomber streams, the B17s could reach 35,000 ft, due to the superchargers but on the bigger raids they were streamed at a variety of levels and the trails (as you say possibly due to the superchargers) were just as much in evidence lower down as at higher levels.
In the early 1960s when the 707 and DC8 were the obvious aircraft to look for on the front of trails, the odd DC6 or Connie could be seen with a very short trail.. The oddest thing I ever saw at the front of a long trail in summer was a Shorts Belfast over Lancashire in 1969 slowly drawing a line in the sky which must have been all of 60 miles long - and at FL230!