I think that's BS. They shorted us on the complete Marshmallow update and now I'm stuck with it. If I had received the 0.1 of the update, it wouldn't bother me so much, but I didn't and there quit a bit of stuff that comes with that minor update.
As for the ROM, well, I'm not sure the phone is worth the headache to do it. Not only that, make a mistake, you become the owner of a brick.