I cannot believe that Moulinsart (or the Herge Foundation) isn't at least contemplating a new adventure or two.
For reasons pointed out before, Moulinsart doesn’t own the rights to Tintin; they exist to exploit rights which have be granted to them by Mme. Rodwell, covering the existing art and stories. A fine distinction, given that she also owns Moulinsart, but an important one.
It is Mme. Rodwell who has the final say, and she has shown no indication of doing anything other than honouring what she sees as Hergé’s wish that the series die with him; it’s not to say that she couldn’t or wouldn’t, but I doubt that something like the movie is likely to change her mind.
even if the copyright holders and Herge's estate agreed to publish new stories, could they legally get away with ignoring Herge's personal wishes or could they face prosecution?
Of course they could - who would prosecute them? The legal rights lie with Mme. Rodwell, and the buck therefore stops at her; it may be that the charitable foundation she has set up might have some influence on what can or can’t be done (e.g., she might have signed some rights over to that to protect them in some way), but other than that, she can do what she wants.