Does it have access to a passport ID? Image database?
How is the one verifying the validity of the certificate supposed to check if it's actually the holder of the certificate standing in front to clear admission?
Your granny doesn’t need to verify this, these are used by employees of venues that want to limit access to their facilities to people who are either vaccinated or tested negatively.
I'm pretty sure the guard doesn't give a flying fuck about my personal information, just like the programmers - so how do I verify myself? Or am I to stay at home forever if I care about my privacy? The EU said very different things about these issues, is that forgotten now? The same goes for the other identity-related EU initiatives, where did all the talk about privacy go? Was it just propaganda, because it certainly seems so now, as there are so many so obvious loopholes it can't be an accident?
If you or your granny aren't good at reading code, you just have to trust that other members of the public who are would have made some noise in the media if something fishy was going on.
Yes -- just like you've always been, long before this pandemic. Every time you're out in public, you're out in public.
You'd still end up comparing a picture to someone's face, though, so you can't really remove the middle man without going into some dangerous facial recognition tech.