You are confusing two completely different subjects. If you are talking about inviting someone for a Shabbos meal as being the meaning of “welcoming”, then no we are not too welcoming. It is O.K. to invite someone to a Shabbos meal even if they turn out to be a goy.
If you are talking about accepting someone as a Sofer Stam, the halacha is quite clear that you should only be buying from someone reputable and should not be looking for a bargain when purchasing tashmishei kedusha. In general, there is a weakness in the entire system of checking out people who write Sifrei Torah and Mezuzos. Yes, we should learn a lesson and require background checks that are published online for every Sofer, and we should require that the name of the Sofer as well as the date written should be information that is included with every pair of Tefillin and every Mezuzah. Otherwise, when something like this happens, the people who were tricked into buying pasul tashmishei kedusha may never even know that they are using pasul mezuzos and tefillin. Theoretically, we should be able to issue a “recall” of all the tashmishei kedusha a Sofer has ever written.
If the Hachnasas Orchim could be more organized, that would make it easier to spot fakers early. What if there was a registry for individuals interested in being Shabbos guests? The first time they wanted to be a guest, they could be invited to the house of a Kiruv Rabbi who checks them out. After that, they might be invited back and they might not. Usually Kiruv Rebbeim can figure out who is problematic and who is not. It might take more than one Shabbos meal, but eventually they will figure it out.