Sorry for the delayed response. Generally Holt flare brackets were just stuck on the underside of the wing and weren't retractable. There was a reason for this - Holt flares when lit were very dangerous and were known to ignite spontaneously. Aircraft which landed using them frequently had to taxi around the airfield until the flare extinguished otherwise there was a high risk of the flame from the flare catching the wing and setting it alight. RAF practice was to fix the brackets down when they were needed and at no point have the flare in contact with the wing. I am sceptical about the emergency flare housing explanation. Emergency flares would be housed in the cockpit where it was easy for the pilot to reach them not under the outer struts. Your point about the inner struts is a very good one and a more detailed look at the panels suggests access panels for the Frise aileron control mechanism. These were controlled via the lower wing and the location of the panels would be appropriate for this.
Anodizing was done at the factory. The first series Gauntlets weren't fitted with anodized panels but once the decision was made to abandon polishing it was agreed to anodize the metal panels and Glosters would have done it as part of the production process.