Hi luna66 - your post got missed.
The thing is your previous regime of 100 mg for 10 days is way below the licesned dose so really perhpas not surprising that eventually your womb lining thickened. The licensed dose if 200 mg for 12 days and evene then with higher oestrogen doses it might not be enough for some women especially if taken orally. I know gynes prescribe lower amounts to prog intolerant women and as such are monitored - so in your case it is good to have discovered this sooner rather than later.
Did you have any other scan from the start of this regime until you esperienced the very heavy bleed? If not these are recommended for off-licence regimes and especially for less than licensed doses of progesterone.
I am so pleased to hear that your osteoporosis has improved since starting hRT and of course you will want to continue but without the worry of thickened lining polyps and very heavy bleeding.
I presume all was well re the plyp and lining and maybe you had a biopsy too to check or maybe they will biopsy the polyp? As you lining is already thickened then you will probably get bleeding on the Mirena as your lining settles down and thins but as the gyane says after the first 6 months there should be no bleeding at all as you are post-menopause. The only reason you had regular bleeding was because you were taking cyclical HRT so once you are post-menopause and on continuous combined HRT ( which the Mirena is) - there is no regualr bleed (once the shedding is complete).
Sorry I have no experience of the Mirena but hopefully others will be along. There are lots of threads on this too if you go to the home page of the forum and do a search.
You might have to grit your teeth and bear it - but think of the benefits to your body and walk tall

All the best
Hurdity x