Sometimes even though a tooth is cleaned out to start the root canal, pain can still persist so further cleaning is necessary. Not all first time open and drains and pulpectomies are successful at taking care of the problem on the first try. Often the bacteria are in the bone so even cleaning out the tooth part doesn't necessarily mean everything gets better right away.
Asking a dentist whether they think another dentist did anything negligent is like opening a can of radioactive flesh eating space worms.
For what it's worth, nothing that you have mentioned so far has set off any reg flags for this dentist. Nerves can be funny things. They may be fine for years and years, but the instant you do something new around them, they can suddenly abscess and die in protest.
I have found some resin cements more irritating to nerves than others. In particular, I remember having consistent sensitivity problems after using 3M Rely X ARC cement. After I switched to 3M Rely X Unicem, the problem disappeared. Usually a nerve is capable of surviving some irritants including new preparations and irritating resin bonding agents. Other times the tissue dies and root canals are needed unfortunately.
I haven't done any zirconia bridges, but the milled zirconia crowns I have done are very, very strong. In fact, they are incredibly difficult to create root canal access openings through. From that fact, I think they should be able to handle having root canals done through them. Your dentist probably would be able to tell you better since there are also different brands of zirconia crowns as well. The ones I've done are Procera and Cercon single unit crowns.
I don't think they are quite as strong as conventional porcelain fused to metal bridges, but the material is quite good.