Does anyone know if when I am upgrading defenses, are the upgrading towers, barricades, etc taken out of commission until the upgrade is completed or are they used at the lower (prior to upgrade) level?
Yes they are taken out of commission until you finish the upgrade: if you are upgrading for example a barricade from lvl 10 to lvl 11, the other players will attack a barricade lvl 10 and NOT a barricade lvl 11.
When the upgrade finish and you will collect it, other players will attack a barricade lvl 11.
So they are not taken out of commission entirely (taken off of the defense grid) but used at the lower level until the upgrade is complete?
That’s totally correct.
the only time a tower is not on the grid is when it is first built (level 0 --> level 1)