When you say " the tail between the fifth and sixth "rings" (just above the area that is no longer viable)." I have to assume that this new wound is above, or closer to the body, than the main wound which is the hardened-off section at the end.
Autotomy, the automatic dropping of part of the tail, happens at locations along the tail called Facets. Factets left by dropped tails on the end of the living tail, at the end of the 'stump", after part of the tail is ejected, are clean breaks without broken and projecting vertebrae or lots of bleeding. Facets are the best possible tail-loss wound, which make healing and re-growth happen most quickly and safely.
For your iguana, the best place for the dried out section to autotate (self-separate) is at a facet some distance above the dead part, in healthy, living tissue.
It sounds to me like that is what is happening. The non-ascending gangrenous part of the tail is starting to drop off. The new crack s the first sign. The red color you report is the normal color of the flesh within a tail. To me it sounds healthy. If it were grey, green, or yellow, that would be an emergency.
A vet can use his fingers to snap the tail off at that new facet cleft (crack). You could do it too, but for inexperienced people, it's just way too gross and scary.
You must make sure that the infection hasn't invaded the flesh at the new crack. If it has, the tail just above the cleft would go soft and mushy and get another color. If that area is firm and healthy feeling, then IMO the tail needs to be completely separated soon at that spot. IMHO your vet should be willing to do that. It only takes five seconds to bend a tail like that until it breaks at it's pre-chosen facet. Right now the inside of that crack is a prime breeding ground for bacteria.
I'm glad you have money for vet visits. IMO vet money plays a vital role for keeping our iguanas healthy. Without vet money eventually they get a problem that kills them.