Chi chi, are you still producing your own hormones?
I've had scans at different times of the cycle and they have varied from 2mm (the latest one I had just after a period) 5mm mid cycle(ish), peaking at 6.3mm right at the end of the cycle and before the bleed. There is a huge variation and some women never get a very low reading even when they use the right amount of progesterone. As always, we are all different.
Someone kindly forwarded this link to me. She is on the same regime as me, although she takes more Utrogestan that I do, and her readings are always between 6.5 and 7.1.
http://sogc.org/wp-content/uploads/2013/01/gui249CPG1010E.pdfTop of 2nd paragraph shows the figures for women on HRT and goes up to 10 - 11mm for women on sequential HRT.
Basically, you are below the level at which they would worry there is a problem.
I do agree that it would be worth trying to have a good clear out before you have the coil fitted. Once you have the Mirena fitted, you will shed automatically if you get a build up.