mgaved first of all: did you test the v3.10.0-dev image? Some feedback would be very valuable.
(a) Do you have an idea of when v.3.10-0-dev will move to being a stable release ?
I've planned to release it in the following weeks.
(b) In the meantime, would using an external USB WiFi dongle and MoodleBox v3.9 solve the problem by handling WiFi traffic on the USB WiFI instead of the internal Raspberry Pi WiFi radio
This could work, but the software installation (driver) could be a nightmare, and some hardware models do not work easily. Eg. the very model you're suggesting (TPlink WN725N) has problems as a AP. That's precisely why I've no solution yet with external hardware.
For another workaround, with an external device, see this discussion.