Can you directly answer whether your tool generates tagged PDFs?
Of course, you can't guarantee that the resulting document is 100% compliant because you can't enforce that the input is valid, but are you at least outputting a complete tag tree with as much semantics as possible given the input?
pdfHTML from iText does it (you can create PDF/UA-1, PDF/UA-2, and PDF/A documents from it), without using any external engine. You can even add your custom processing.
Of course, you can't guarantee that the resulting document is 100% compliant because you can't enforce that the input is valid, but are you at least outputting a complete tag tree with as much semantics as possible given the input?