Re seat post slipping: With an aluminium frame, the metal being soft "rubs off" easily and provides for a certain amount of "lubrication", or at least smoothing out any microscopic bumpiness which might otherwise have provided extra friction. So the seatpost ends up getting more slippery all the time as raw aliminium rubs off on the seatpost. Cleaning the aluminium seatpost very thoroughly to get rid of any rubbed off aluminium from the frame, and then claming the clean post securely should cure the problem, but friction grease will definitely cure it.

This effect was very evident on my Swift with carbon-fibre seatpost: It would slip and silvery aluminium metal would stick to the seatpost. This felt very smooth and slippery to the touch. Friction grease solved the problem.

Steel doesn't have this problem.