During an offensive, is it true that all activation paths are traced before any unit moves...
This is correct
but supply for any particular unit that has been activated
is traced the instant it moves?
This part is incorrect. Supplied status is a pre-requisite for activation, and is not checked again during the offensive.
if the activation path to a unit is blocked by an enemy ZOC, one can't activate a second (air or carrier) unit, move it to neutralise the ZOC, then activate the first unit in the same offensive.