I don't have my books with me so I can't check on what the table was made out of, but if it was wooden, it would likely collapse with the weight of a ceiling's worth of coins on top of it, and/or the coins on the floor would gradually slip under and both crush him and suffocate him. It simply beomes a question of how he'd rather go. We are talking about over 45 metric tonnes of metal here.
The point the books keep making about Alex is that he is exceptionally good under pressure and when he is endangered, if it could have saved him, he would have thought of that; It is likely that Roper simply panicked and couldn't think.
I think the finger prints thing is less of a mistake and more of an incomplete description of his methods in the text. If one has managed to get the finger prints of the president of the USA, one would certainly manage to think to use them as a mould rather than as a stimulus themselves. Although yeah, it is a bit lucky that he managed to get a whole print of his fingers.