Dont ever defend your HQ in skirmish. Game will glitch on deployment.

Ive done defend HQ mission but not on Steam and not in skirmish. But you dont have the normal deploy zones during this mission. Rather individual predetmined spots are scattered around your HQ which you can assign your gangers to.

Attacking party has the normal deployment zones. Are you sure you were trying to deploy in valid locations?

Yes, HQ Assaults/Defenses are special modes and you can't deploy in the same way than on usual maps.
Also, you can't play them on Skirmish as it's only available in Operations.