Liz,
Having had a similar experience I can relate to your (or should I say Robin's) pain. I too was not Dx'd properly or refered by my dentist even after I questioned him about the tumor. But for the grace of God I survived anyway even though it was quite advanced when I finally went to an ENT. I had been a patient with my dentist (recently retired) since he started in practice. Their office has only seen 3 patients with OC in the past 25 years (and they have a LOT of patients). I was very conflicted over how to handle it and decided to let it go. He wasn't responsible for my cancer and had a reasonable explanation for his failure to detect it.

I also have to take some responsibility because my own fear crippled me from getting a referal to an ENT sooner. I knew something was up and I stuck my head in a bucket of sand like the proverbial ostrich. I also went to an GP (twice) and he didn't refer me for quite some time as well.

From reading the report it sounds to me like you may have material for a strong civil liablity suit.

None of this will bring Robin back and may have consequences of setting your grief recovery back for some time, reliving the horrors of Robin's circumstances in court.

Robin was young and had many years of earning potential taken away from him (and you). This is the typical basis for civil liability claims in the US. The burden of proof is much less as well in a civil trial.

Only your heart knows for sure what path you must take.