Hang in there. At five weeks, even with that high of an hCG level, you aren't going to see anything other than the gestational sac because bubs (or bubbies) are just too too tiny to see. As long as there is growth, there's hope and if your doctor is thinking all is well, then I'd go along with it. And with twins, there is usually a much higher hCG level than with a singleton pregnancy so I wouldn't worry about the levels. I'm sure if you were to have another u/s next week, you'd see even more. At this point, even a day can make a huge difference.
As for figuring out when you got pregnant, I'm assuming from your ticker that you got pregnant right after a miscarriage and didn't get a period first? If so, the exact same thing happened to me this time round. M/C in early Oct, BFP 5 weeks later. My former dr put me at 5 weeks but I knew (based on opks) that I was more like 4 weeks. My first ultrasound was 7 weeks after my miscarriage but baby measured 6 weeks (which coincided with my pos opk). The thinking is that my hCG dropped back to 0 around a week post-miscarriage and then I ovulated 2 weeks after that. It did add to my stress with the first tri to know I was pregnant again so soon after back-to-back m/c (I had a loss in July as well) but despite it all, I'm 21 weeks now and baby is fine.
I know it's hard to be in limbo right now but try to relax as best you can. Right now you are pregnant, all looks good, and you are going to have twins! So just breathe and pamper yourself a bit.