Sadly to say, there are a lot of biological dentist/holistic dentist out there who ONLY aware of mercury/amalgam issues, but have no idea on root canal & cavitation issues. The problem when you come to these dentists, they would prioritize to take out the mercury first. However, if you have root canal /& cavitation issues, you need to tackle those issues first before removing any amalgam. My suggestion to you is to find a good holistic/biological dentist who understand the root canal & cavitation issues.
I know it's such a hassle. I am also facing the same thing. I had to fly 12 hours to another country just to meet the dentist who knows this (not easy: I have list of health problems as well). And the treatment is long. I am just started though, so don't know whether this dentist approach will work or not. But he seemed to know better than any other dentists I'd seen. You can contact DAMS for list of biological dentists in your area (http://dentaltruth.org/). After you get the list of dentists, you can call/email them to ask whether they know about safe root canal extraction procedure & cavitation issues or not. If you do not get good answer, need not to waste your time and money going to them. Narrow down your list and go only to those who knows. Hope you find good dentist.