Should I log an Amazon Flex block as one shift?

One block, one shift. Use the times you actually started and finished, not the block's scheduled hours.

That difference is the point. A block scheduled for four hours that took four hours forty is a shift that paid a fifth less per hour than the app implied, and logging the scheduled time hides exactly the thing you want to see.

If you do two blocks in a day, log them as two shifts. The mileage and earnings are separate and averaging them together loses which one was worth doing.

Business miles run from the depot to your last drop. Home to the depot is commuting, so it goes in the non-work miles box, where it still counts toward fuel but not toward your claim.