Can Clan Castle Be Used While Upgrading?

During the upgrade you can use the Clan Castle without any issues. You can request troops, use on attack and defenses and collect resources from the Treasury.

Can you use troops while upgrading?

To actually answer your question, yes. Even as a unit is upgrading, you have the authorization to train in your barracks. However, until the upgrade is complete you will only have the current troop level. Once the upgrade is complete,you have your troop upgraded and you will be able to use it at that level.

Can I upgrade clan castle during war?

This is from the Clan Wars FAQ, written by a SuperCell employee: The war base can not be directly upgraded or rearranged, but will instead mimic the arrangement and highest completed upgrade levels of your village during the preparation day. Buildings and traps will never be “under construction” in your war base.