I dont think that many Players gem hero Upgrade time. May you use some gems, if there are only a few hours left and it is cw, but nobody gems 5 days every Level on both heroes.
On the other Hand Supercell looses masses of gems for boosting, if Players dont farm while their heroes are upgrading.
I think the whole idea of having the heroes unavailable during upgrade was wrong from the start, not only because of clan wars. This is a troop upgrade. You dont loose all your giants for 10 days if you upgrade them to lvl6, so why do this for a unit that is upgraded far more often.
The idea of the 1 week upgrade them and using a builder is good. Heroes should not block the lab and should take a while to upgrade, but they should be available for offense. Maybe just disable them on defense like any other buildings.

