That is unfortunately the way of things, was the new build a private inspector?
Again was this done without any reference to any guidance. Did the instances you refer to follow the guidance.
New build, afraid I couldn't say, but given the complete lack of H&S during the build, and the poor standard of the finished build, (just from an external viewpoint), I would have said either Stevie Wonder has passed it, or they've paid someone. Things range from same section of sewer keeps blocking, to missing roof verge sections, gable covers, uPVC doors that wont stay shut, rainwater overshooting/missing the gutters, downpipes coming apart/falling off the wall, and whichever Muppet managed to design it so they had a BT Cabinet in the middle of one of the entrances to the parking area, (which has subsequently been blocked off...), needs to also go to Specsavers. Heaven only knows about the internal snagging lists....
I have not idea whether the alterations to these drainage systems followed any guidance, I only know we come along to jet the sewer, usually as planned maintenance where there is a known issue, (fat build up/poor gradient/root ingress/something else), to try and prevent future issues, having no previous history of issues, only to find on this occasion an irate Householder who's just had their bathroom redecorated.... Then have the reams of paperwork and reports to do to explain how/why it might have happened, and all the preventive measures that need to be put in place to prevent a repeat performance by the next crew who attend.
My point is, advising people that it's OK to remove an existing open vent and cap their soil pipe with an AAV, with no knowledge or sight of their system, is very unwise. It's DIY'ers and bodge artists, looking for easy solutions, and most with little or no knowledge, (or care) to how a drainage system operates, that are causing the issues, and usually only find out when some sort of pressure buildup has resulted in a rather unpleasant eruption from the drains in some way shape or form.