3 surface filling billing

Yes but in many cases the insurance will downgrade it to a D2393 and pay it as that code. I've recently dealt with this with a patient with Guardian insurance.

As long as the decay does not extend or connect to the buccal surface, you are coding correctly. Sending an x-ray can support that you did code the procedure correctly and you are not bundling. Insurance companies typically approve a 3 surface benefit in these cases.

Yes the insurance may pay it as billed or will down code it.

It is inappropriate and fraudulent to code two separate fillings on the same tooth when a more appropriate code exists. This kind of billing is called unbundling and is considered fraudulent and abusive. Rule of thumb: always code to the highest level of specificity, meaning if a more appropriate code exists use the more appropriate code at all times.

Yes, for example if it is a composite filling on tooth #19 DO and B - you would code as such:
D2392-19-DO
D2391-19-B
Most insurances will down grade it to a one three surface filling. If you are in-network you will have to adjust the difference off. You cannot help how insurance companies process claims. No matter what always code exactly what you do.