Siyyum Question

I've recently completed a new Sefer Torah and will be holding the siyyum in early June.  At the request of the shul I've left quite a large number of letters open so that "as many as wish to may participate in the mitzvah." My concern is that given the large number of people called up, many, perhaps most of whom I will not know, that among these individuals could be invitees who are not shomer mitzvos, or perhaps even halachically Jewish.  Does anyone have a politic way of allowing people to 'participate' in the filling of the letters without running the risk of rendering a year and a half of work posul? 

Also, what would be the correct course of action if it did come to my attention that one or more letters had been written by someone who is posul for writing, but after the fact I had no way of knowing which ones? 

Would it be permissible to use an ink for the siyyum, dyo l'netzach for example, which before it has fully cured can be peeled off the klaf, and so simply remove the filled in letters and re-write them myself later?

sha'ala on kof

I was sent some Mezzuzah's to check.  Oddly enough one was still new in a plastic wrapper with this hekhsher on the back:

However upon inspecting it visually these were some of the more glaring issues that I found:

Does anyone know anything about this Hekhsher?