White Ark

If I have this Base and GD04-006 V-Dash Gundam in play but no other Units, can V-Dash Gundam rest this Base instead of a Unit and activate its 【Activate･Main】 effect?

No, it cannot. When you would rest a Unit, this Base's effect enables you to rest the Base instead, so if the prerequisite Unit isn't present, the Base cannot be rested in place of it.

if I have this Base, GD05-001 V2 Gundam, and two other (League Militaire) Units in play, can V2 Gundam rest this Base instead of the two Units and activate its 【Activate･Main】?

No, it cannot. It can only rest it in place of one of the Units.