I would be inclined to just give the system a good flush through then after manually latching the diverter valve in mid position refill it very very slowly by tying up the ball cock to let the water trickle down through the F&E tank and down through the cold feed, this should/may allow air to vent up through the vent and still allow water down the same vent pipe, thats how I refill mine but I have a service 1/2 ins isolating valve on the cold feed to the F&E tank, I have no AAVs anywhere in the system and can run the circ pump at any speed I like with never a sign of air. Maybe also blank off the vent as suggested even if only temporarily.
Your system sounds as if still full of air but you are getting no proof of this when venting/bleeding the rads, weird.
Your system sounds as if still full of air but you are getting no proof of this when venting/bleeding the rads, weird.