This is clear and thank you for clarifying! A bit shift modifies possibility by 50%.
Yes, but the hash target doesn't necessarily move from 135 bits to 136 bits, it moves from 135.1 to 135.4.
Just a question if you know for sure. When calculating the total difficulty of a chain we count the values of the actual hashes or the difficulty targets in the blocks?
The total difficulty of a chain (for the purposes of determining which chain is longest) is the sum of the difficulty targets, and has nothing to do with the "luck" of the miners / block hashes.