"The pace should anyway be slow enough that people can join us on foot / scooters / skateboards / rollerskates if they can't bring their bikes."
I think if enough people turn up (should be over 1000) pace will inevitably be fairly slow, though presumably not walking pace all the time!
It would be simpler to assemble in The Meadows, go up MMW and all the way down The Royal Mile.
This is as 'city centre' as is practical at the moment.
I suggest there are too main aims on the day - get a lot of people to turn up and get some good press coverage.
It might be able to get the police to agree to riding down both sides of George IV Bridge - that would make a good photo.
The route would take in the pedestrianised bit of the High Street - showing what can be done when more attention is paid to cyclists and pedestrians.
It's important to highlight that all this is not 'just about cyclists'. 'We' want safer roads and a change in priorities for the way streets are used and various users favoured/prioritised.