First, measuring at 6 weeks is often too early to actually see a heartbeat. It's a bit hit or miss at that stage.
Second, It's really not accurate at all to base it on your LMP.
Everything about your posts says that you ovulated later than the
"standard" 14 days which dating by LMP assumes.
Instead, let's adjust how far along you are by when you got your positive. In general, the earliest you can possibly get a line on a HPT is 7 days after ovulation (most don't actually get it until 10-12, but we'll be extra cautious here). At that point you would be 3 weeks pregnant. That would then make you 7 week and 4 days pregnant today. So really, you should be seeing a heartbeat, and measuring +/- 7w. HOWEVER, I can name a few stories of women i know and trust from these boards where they were measuring up to 2 weeks behind (knowing when they ovulated, etc) and everything turned out ok. One of them has a 5 weeks old in her arms right now. Pregnancy can be weird, and not all are text book cases.
The protocol to confirm miscarriage should be to have a follow-up ultrasound a week later to
see if there's any growth or not. I would never suggest intervening
until you're 100000% sure. Your doctor can also run repeat HCG tests to
see what the numbers are doing. If they are decreasing, then sadly you
can be sure you have miscarried.
So my advise, wait for either HCG test or a follow up ultrasound before proceeding with a D&C (or cytotec, or waiting it out naturally - you do have other options) to confirm. I I think you should hope for the best, but expect the worst. At this stage, there's still a small chance that things could be ok, and the one thing you never want is to wonder, "what if"
I'm so very sorry this is happening to you. I've been there and I wouldn't wish it on anyone.