The overtext below is true.
The overtext above is false.
I assume that once cards have been placed to form an Agent, the order of cards in the stack is then fixed.
If you move an agent to an event which already has an agent, do you place one agent on top of the other to form a single agent, and if so in what order are the cards stacked?
Nope. You then have two agents at the location, each of which can take one action per round. This can be very powerful if you get the timing right.
Can the order of cards for an agent be rearranged at any other time?
No. Once the agent is in the past, they have a fixed schedule.
Just checking the components: 2 decks of cards, one set of which forms the event cards leaves 7 of each number.
Correct. 70 card deck, plus 10 for the timeline.