Yeah thats pretty frustrating that its hard to demonstrate the link between root canal therapy and certain chronic diseases. My xrays looked absolutely normal and went to two conventional oral surgeons and they told me the same thing, that they can't remove these because there's nothing visually wrong with them.
Well when I got them removed, it had a large white cyst dangling in one of the inside roots. Even if your immune system is overworking it might not greatly impact any readings on the blood test; this is a slow progression to develop chronic diseases so unless your system is too tired to fight you might start seeing some anomalies in the tests. Not sure what other tests can be done to give you a better clue, but having dead tissue in your mouth harboring harmful bacteria is certainly harming you (to what extent is the question); no where have you heard anyone for example get a finger or a toe badly severed and started to decay with bacteria and not having it to remove it promptly or else it could spread to the whole body; Teeth are no different, theyre still part of your system.
Good luck
Mike