You should ask the doctor about
"My concern is, that I normally go about
10 days between BM's, and I'm approaching that soon. I'm worried that starting the Sitz mark so soon to my next expected BM will influence the test incorrectly..."
and see what they say.
I would also discuss with them ALL options you may have before surgery and maybe even a second opinion. Of course you have to wait for the test results as well.
Sorry to hear your struggling through this and hopefully they can sort it all out for you.
I am not a doctor. All information I present is for educational purposes only and should not be subsituted for the advise of a qualified health care provider.
Please make sure you have your symptoms diagnosed by a medical practitioner or a doctor.