EWL,
I only know you from your posts, but have been reading them for a while.
Whilst I understand your concerns, your actions to me seem reasonable, and from a follow-up servce point of view, above and beyond the call of duty - frankly, I think you are probably beating yourself up over the issue which is really not of your making.
When it comes down to it, you are providing a service - an excellent one by the sounds of it - that any customer is entitlled to purchase, regardless of their personal eccentricities. (To exclude the sale of our service would from what you have said have no legitimate basis - exactly why the lady in question is persona non-grata to the other agencies is not disclosed).
As you have eluded to - YOU have not let anyone down. [The state of medical care in Australia since the mid 1980's for the mentally ill or impaired is largely to blame, as well as societies general ignorance of the problem.]
I don't mean to piss in your pocket, but the fact you even feel responsible shows that you care about your product, your service and your customer, and that eclipses 90% of the travel industry I see around me.