Re RCT is possible but prognosis can be given after clinical and radiographic examination.

Yes it’s quite difficult to do re rct again but you have go through again since you having infection but surely you will find after re rct And whatever sealant materials been used in root canal feeling that has to remove completely but that’s totally task of the dentist

Yes ,its very difficult but have to be done again.

Re RCT is definitely possible whether it is a molar or a front tooth. But one thing that I would not agree upon in your description of your problem is that the filling material used for obturation in root canal treatment is non-resorbable by action of any body enzyme. So it is more likely that it was not obturated properly in the past and that could be the reason for the infection. And this is a tentative suggestion based on your description rather than a look at the clinical condition of the tooth or the X-ray of the tooth. You can post the X-ray for a more rational opinion from our side. Regards.