Return of dental issue that multiple dentists have worked on. If new insurance denies coverage (“pre-existing”), can I ask the dentist from my previous insurance to fix it free of charge?

Rewrite of an earlier post where I also asked if I should fudge the date I noticed the issue return, have been informed that that is insurance fraud lol so will no longer consider that. Other questions still remain:

I have an issue with a filling that falls out every few years – have had 4 dentists replace it at this point (due to being on 4 different insurances over years) and they all told me it wasn’t an issue with my dental hygiene or health, but that the last dentist didn’t do a good job. (Seems unlikely all 4 dentists did a bad job, but possible.)

In June, this filling fell out again while I was eating. I did not have dental insurance at the time (couldn’t afford it, job didn’t offer good plans), and there was no pain or discomfort – but I planned to enroll in dental insurance next period to fix it.

As of September 1st, am officially enrolled in new plan with okay dental coverage. Tomorrow I will be going to an in-network dentist to fix the filling.

There is still no pain or really any other symptom (not even sure if the entire filling came off or just part of it), but I am worried that this time the fix won’t be a minor procedure (replacement filling, like the past 4 times) and will be major (root canal, etc).

I can cover a minor procedure out of pocket if my claim is denied due to it being “pre-existing” – but I cannot afford a major procedure out of pocket. A few questions:

Question 1: The problem is “pre-existing” as it predates my insurance start date. However, would the start date of the dental condition be June (when it most recently fell out) or is it dated to the last time I had the filling replaced (under a different insurer and dentist)?

Question 2: If it is the latter, can I get the most recent dentist who replaced the filling to do a major procedure for free/at reduced cost? Even if they did the last replacement under different insurance & are out of network for me now? What complications might there be in trying to get a former dentist/insurance to take responsibility for it?

IMPORTANT NOTE: If my claim is denied due to it being pre-existing but the procedure is minor/affordable for me out of pocket, I will pay to avoid hassle. Not trying to pinch pennies, just want to know my options to avoid debt.

TL;DR If current coverage denies claim for procedure to fix recurring problem, can I ask the last dentist that worked on it to cover the treatment even if they were under a different insurance plan?