I did cancel the reservation (made it this morning) and booked the
last flight at 9:40pm. So now, our layover will be from 7:35pm to
9:40pm. The British Airways agent affirmed that this should be enough
time. Would you agree?
Is there any reason to be concerned re: customs, since we'll be in
three different countries on two different airlines that day?
2 hours is not enough time.
This is NOT a connecting flight. You will be taking 2 separate airlines that have no interline relationship. This is a stopover.
On two separate airlines, you are taking a risk with only a 2 hour stopover time that you will miss your 2nd flight. If you are prepared to buy a last-minute ticket at CDG for your flight to London, then the only risk you have is making sure there are actually seats available on a later flight. Otherwise, allow 4 hours, not 2 hours especially if you have checked luggage that you have to retrieve at CDG first and then check in at the BA counter BEFORE waiting on the long security lines and passport control. Delays can always happen.
When I travel on 2 separate, non-interline airlines, I allow a 4 hour cushion at the stopover airport because delays and problems happen quite often. I typically have stopovers at Heathrow or Schiphol.
Customs is only an issue if you are either smuggling contraband or have purchased items to declare.