And the 'eg' also gives leeway for people who might have very good evidenced reasons for not quite attending as frequently, such as having treatment for cancer/being in hospital, being a kinship carer for a lot of children/another child with SEND or illness, etc.
Generally, Priests don't have time to be faffing around writing letters as they'll have requests for hundreds of the things, so they complete a form. Essentially, it's a case of confirming yes, they have heard of the kid and their parent, they are catholic, they have been to Mass more than twice before submitting the application if there is ranking on the basis of regular attendance in the policy, etc, and gives the opportunity for the Priest to confirm if there are additional circumstances that they would like to be taken into account if possible.
The person dealing with the applications just wants all the information and evidence they need to put them in the correct category and ranking so the ones open to interpretation can be put forward to the governors in an anonymised format, the decisions of that meeting can be recorded, acted upon and then the rankings uploaded to the local authority so they can offer places in accordance with preferences. Having to deal with 1250 applications for 120 places in the space of about four weeks, wrangling the LA applications portal (which is usually a PITA before you take into account people not typing their child's date of birth, not leaving contact details, giving a different surname on the two systems, etc, etc) and all the queries whilst also doing at least one other job throughout means they don't have time to be policing anything, other than 'do I have the evidence? What category is this?' and moving on to the next application.
Thankfully, the majority of applications are cut and dried. They have Baptism certificates, First Holy Communion if it's secondary, priest references - tick, tick, click, enter the category code and done - or Christening certificates/confirmation from religious leaders for other faiths, enter the category code and done. Because that gives time to look at the exceptional cases and make sure they also receive fair consideration. The unlucky Admissions staff who have to manually input paperbased applications are under even more pressure because of the vast amount of time that takes in itself.
It's a complex process with strict legal requirements and time limits. And technology, which is usually the biggest pain of all. There's just no time to be arseing about with the sort of shite the OP's MIL is running her gums off about; it's ultimately the Governors' decision, but for them to make decisions in time to comply with the Law means that a pretty low paid grunt has put the hours in to keep their Committee meeting to under 97 hours long.