I'm really sorry to hear about your husband, you are going through a very difficult time and this is an added problem I'm sure you could do without.
First of all, you finished the norethisterone on 17th March and you have started to bleed now so chances are, this will become a full scale bleed, possibly very heavy, which will shift a significant amount of that build up. Once you have finished this next bleed, that would be a good time to have another scan to get a clearer picture of what is going on. Obviously that is a very thick lining measurement but to put things into perspective, 11mm is often the maximum for pre-menopause women so you are slightly over that - more than 14mm is considered thick. This is a very common for women your age when hormones start to go out of sync and you have more oestogen than progesterone. It is easily fixed with a high dose of progesterone which you have already taken and if that doesn't shift it to an acceptable level, you might need another dose.
By the way, it is normal for breakthrough bleeding to temporarily stop when you start taking progesterone and then start again shortly after you finish taking it.
At lot of us on here don't like norethisterone but it works very well at womb lining clearance so you are on the right track. What you need to do now is to find an HRT regime that keeps the womb lining thin and stop this from happening again. It sounds like you tolerate progesterone well so this should be relatively easy.
Hopefully you can get some expert advice from a menopause specialist.
Please keep us updated.