Robert,
That is one First Class letter !!!
Not surprised you are batting a thousand with it.It covers it all.
Don't believe anything the collection agency tells you.If their mouths are moving,they're lying.
Nothing counts from them unless it is in writing.They got you mad and upset,that is exactly what they are trained to do.
Keep every piece of paper concerning this,forever.
There is a fair chance they will bring this up again in the future.
Again,the Dr. can put a stop to this.I'd be beating on his door,in person.
You have to take charge of this.