Could one really get ice accumulation on the airframe in the range +4C to 0C, at typical GA airspeeds (below say 150kt)?
Yup, the FAA says so, therefore it has to be true.
Actually you seem to answer the question yourself a bit later:
Perhaps the other thing worth mentioning is that even in clear air, the pitot tube can freeze up below 0C, so pitot heat should be used anywhere near freezing. I've seen this myself, and put the pitot heat on below +5C as a habit.
TKS is a get out of jail card.
I have had a few encounters with icing one of which is a 'known icing certified' medium twin. Scary stuff.
FD
PS Thought you would have left the training process long enough now to stop you harping on about the poor quality of the training fleet and instructors. Jeepers.