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I don't think overhead power lines heat up much in normal operation, given the economics involved. They will definitely expand/contact in line with ambient conditions however, and also must be flexible enough to sway in the wind without causing excessive abrasion between the cable and fibre. Edit: the helical wrap would probably expand/contact more than the electrical cable, as it'd be "wider" overall.

If it is wrapped, the spiral form will provide the necessary slack. You just need to wrap tighter in summer and looser in winter when installing the fiber.

This is interesting, I've never heard of this. How much stretching/sagging are we talking about? (I.e. a couple of inches difference, or is it a large amount)