Four hundred million years ago, life was already well established, with a wide variety of fish, and the first land plants. The explosion of the Jagaroth ship should have been set four billion years ago to coincide with the development of life.
Earth's atmosphere 400 million years ago would have been too toxic to breathe. The Doctor and Romana may have been able to function for a few minutes without any oxygen to breathe, but Duggan, being human, should have been shown to be using breathing apparatus.
The Count's time machine, as modified by Romana, would enable him to travel back through time, even to 400 million years ago. However, that would only take him back to the same point on prehistoric Earth where Paris would one day be as (unlike the TARDIS) his machine was not a time/space vehicle. When the Doctor arrives at the same point in Earth's history, a few moments before the Count, he says they are standing where the middle of the Atlantic Ocean will eventually be. When the Count/Scarroth arrives a moment later, it is never explained how his machine transported him thousands of miles away from its origin point rather than just through time.
Romana is wiring a piece of equipment to a UK standard three pin plug but as they are in France she should have a continental two pin variety.