If you're already down to 100 feet or so, the chances are that whatever you do to intervene will not prevent the aircraft from touching down - and probably still very heavily. The difference will be that YOU will have been handling the controls at the time, not the Captain. Guess who then gets the blame in a "your word against his" scenario, where he will claim that he was in the process of rectifying the situation and you made it worse. Maybe the CVR replay will put you in the clear, but maybe not...
This will ultimately depend on the type of aeroplane, it's weight and responsiveness to positive GA control. As has already been said, a high ROD is unlikely to develop below 100-200 feet so you will have been calling anything that is non-SOP well before that point. Thus, hopefully, you won't be having to try to rescue the situation when you're already down among the weeds.
As for lack of proper flare, that's a tough one because any intervention by you could well make the situation worse. Best advice has probably already been given to you... grit the teeth and keep your tongue away from them!