The rule regarding more than one departure and one arrival to a continent is:
4(e) Only one intercontinental departure and one intercontinental arrival permitted in each continent except as follows:
1. Two permitted in North America.
2. Two permitted in Asia.
3. Two permitted in Europe/Middle East for travel to/from/via Africa. If travel is to/from Europe in both directions, itinerary may not include Mauritius/South Africa.
Therefore, either your flight CDG-CPT or your flight CPT-LHR (or both flights) must be via the Middle East
So CDG-xDOH-CPT-LHR or CDG-xLHR-CPT-DOH-LHR or CDG-xDOH-CPT-xDOH-LHR
The rule regarding changes is:
16(a) Rebooking / Rerouting.
1. Before departure
...
2. After departure:
a. Changes are permitted provided ticketed points remain the same.
b. Changes to ticketed points are permitted at a charge of USD 125 per transaction.
c. No Show requires rebooking at a charge of USD 125.
Therefore date/time/carrier changes are free of charge.