Well i doubt if i can further this investigation much more than where it is now, maybe others can come in and give there opinion on this aspect, looking again at those stones, i can't believe that that whiteish dressing is their natural patina .
Of course we can make a distinction between plaster and whitewashing , although they may be based on the same material but the later being highly thinned out.
So logically one can make the case for using plaster on the cracks, but as for the slopping of a further slip around the stones is baffling , as i cant see its worth.
As for how they would gain entrance , well its not so much as gaining entrnce but one of when was the last time they had access , of course its when the last beam was lowered in place, but i have wondered if those beams sit on the end stones at each end of the chamber, or did they slide them in at a later stage.
So, have hit a limestone wall, ?