Bluebellwould, yes I have made the same decision if ever there is a serious diagnosis, so I totally 'get' you. However, with the info you've given I would have the colonoscopy just in case it's something more minor than you suspect, easily treated and maybe not be life-threatening.
Having said that though, I'm a decade older and with other painful problems that I can live with for now, but if things were to go downhill big-time, I will refuse all treatment, it's totally 'your' decision though and nobody can force you.
Not sure how close you are to DC's but I wouldn't share the info with them either...then again, mine are estranged (their decision, definitely not mine) so I wouldn't want their input, been there done that after an incorrect terminal diagnosis some years ago.
Please continue to post so we can offer some moral support as it's not easy having to make these decisions alone, specially after supporting your DH through his journey and knowing what's ahead. I've been there too!
Hugs 