I'm so sorry you are going through this.
Sometimes with a MMC I have spotted and this has led to a natural miscarriage, sometimes I have spotted but it never progressed and I had to have an ERPC anyway, It doesn't seem that one always follows the other, unfortunately.
There is apparently no risk of waiting an additional 2 weeks.
I don't know where you are in the country, but if they are following national protocols, which they probably are, the 2 week wait is to do with the size of the baby- the head to rump measurement at the scan is used to determine whether they can proceed immediately to erpc or if they need to wait and rescan- I can't remember what the measurement is exactly but it is around 7+2weeks - if it is definitely over 7 weeks then they don't need to wait, if the size could indicate it is 6+5, say, it is not definite that the HB would show up until 7 so they have to wait to check, I know the waiting is absolutely heartbreaking.
I have over-ridden it before. I was absolutely CERTAIN of my dates- have a 28 day regular cycle and got two BFP on day 28 so I knew I was absolutely definitely 10 weeks and not 6, and they did agree to perform the ERPC without the two week wait, if you feel you can not wait and are settled in your mind that the dates are right it would be worth discussing with them.
IME whether a MC feels like a heavy period depends upon when the baby stopped growing- I have had one whereby it was just like a very heavy period, but my HCG levels before it started suggested that the baby had stopped developing at 4 weeks.
For me, a MC of a 7 week one is NOT at all like a heavy period- it is very very painful, I pass huge clots and is NOT at all something I could take in my stride, so I would unfortunately prepare for this eventuality.
I needed time off, support, strong painkillers, lots of thick pads with wings (it is dangerous to use tampons even though the temptation is to use them so you don't have to look at what is going on), dvd box sets etc.
Once it started properly it was about 4 hours of bad pain with clots, and then about 5 days of spotting afterwards.