This is a difficult situation. It does sound like it could well be an allergic response... You may want to look at ceramic porcelain, as this stuff is also stronger and harder and does not tend to leach out its components, it is not a plastic either.

You will of course need to consider the bonding/lining agents. It's unfortunate that you are experiencing such reactions, because I'd suggest getting samples and testing them in your own mouth before replacement. But given the fact you are already reacting, you won't know the difference.

Gold is another option, but you'd need to get a pure source.

Glass ionomers could be another option and cheaper obviously. Is there anyway you can test these materials to find out if there is an allergy? Can you find someone who does testing and tell them about the reactions you are already experiencing and see what comes up when they test you with plastics?

I would get in touch with a biologic dentist perhaps about this and ask them what they can do in case of plastic allergy.

Sorry for your situation, I wish I coudl provide the answers.