Incontrovertible points:
Cars entering the roundabout from 12 or 3 o'clock should enter by the inside lane if they are intending on exiting at 12 o'clock (i.e. going fully around or 3/4 around the roundabout)
Cars entering the roundabout at 6 o'clock have the choice of either lane (to then exit at 12 o'clock)
When exiting at 12 o'clock, cars in either lane can choose to exit by either lane. Normally you would stay in the lane in which you have been traveling around the roundabout on, but if you needed to be in the left lane immediately after exiting and the rules had dictated that you should be on the inside lane while going around the roundabout (see above), it is fine to move into the left lane while exiting, as long as you don't cut off someone also on the roundabout and already in the left lane
Blue car should have given way to red car on already on the roundabout. It doesn't matter that red car was changing lanes as they exited the roundabout. That does not negate the fundamental rule that you must give way to cars already on a roundabout