I think this is perfectly normal. Today makes exactly 6 weeks since I began HRT and I have noticed the exact same things you mentioned, including the "marble". I found this on a doctor's website who specializes in treating trans people:
"Soon after beginning hormone therapy (about 2 to 3 months), there will be a nodule-like formation behind the nipple and a feeling of tenderness or sensitivity in the area. This early stage of development is caused by an increase in the ductal system behind the nipple as part of the transition process and will usually normalize in a matter of months."
of course obviously the 2 to 3 months part is a little off; this started for me at about 2 weeks in.
As for the fluctuating areola size, I have noticed this too but I also have noticed it happens for bioligical females as well. From what I can tell with my own experience, it has to do with skin temperature. They (nipples and areolas) seem to be much larger when warm, like when I'm using a heating pad or wearing a warm shirt, and smaller when I'm cool and/or my skin is moist.