I disagree with none of that. All I disagree with is your bald statement that she should have been filing taxes in Trinidad and that should be a way to provide proof of marriage. Unless Trinidad law requires her to file, then not filing is of no moment. In fact, even if required to file and she did not, all that means is a missed opportunity for evidence of marriage. However, since the OP was supporting both, maybe he was filing tax returns there and claiming her as a spouse and dependent. That would be useful evidence. Again, I am clueless as to Trinidad law and maybe there is no such thing there as claiming dependents, etc. Or maybe the OP was not filing, even though maybe he should have. Again, all that means is that there will be no supporting evidence coming by way of what was filed for tax purposes.
It seems to be that, no matter what, it should not be hard to prove marriage and 6 years of cohabitation. If married, as the OP says, surely there is a record of that, such as a marriage certificate. There would be evidence - that could be put in affidavit form - of friends and family attesting to knowing the couple as spouses for 6 years. There should be no shortage of evidence.