Not sure where you got the information about some people having difficulties receiving their letters and they subsequently missed their interviews because they did not know of the interview date/time. In this forum, one person reported getting a call from the IO who asked them to come in same day for the interview. Another person reported (today) getting an update which says they failed to show up for an appointment - as of now, there’s no confirmation if this appointment was supposed to be for an interview (which they never got the IL for), or a system glitch.
USPS has been super slow with mail delivery since last year (thanks to Trump’s lackey, DeJoy came onboard), not just since last month. If you’re seeing mail notifications in Informed Delivery which are subsequently not showing up in your mailbox, that is something you’ll need to go sort out with the USPS. Tiers 1 and/or 2 agents sometimes do not see some IL in the system, because some FOs are not updating the information online. Each FO is responsible for scheduling their own interview appointments, there’s no central body responsible for scheduling those, unlike in the case of the bio appointments which the NBC is in charge of. AFAIK, you can’t simply walk up to the FO to talk to a “receptionist” without an INFOPASS appointment, but if it will help put your mind at ease, you can attempt a walk-in to see if the security guards will allow you in.