The argument about NAIPS' prediction being more accurate is correct, but irrelevant as it is the GNSS unit's own database (as Glekichi has pointed out) that will determine if the unit sequences onwards, not NAIPS.
Its not irrelevant if your relying on using a 145/6 unit as one of your 2 approaches required (CHTR/RPT) or as your sole approach (PVT/AWK) its required. But as someone has already mentioned here, the outages are usually only for 6 minute or so intervals (historically).
I understand the NAIPS FD/FDE prediction is more accurate as it can have info on scheduled outages/maintenance which the units almanac will/may not have.
The receiver will not sequence onto appch (or LNAV) mode if RAIM won't be available, so the idea of doing the manual check is so that you can select a different approach before you even begin, rather than waiting until it's about to transition and being unable to continue.
Exactly! Why wouldn't you do one in flight? Finding out on a 7nm final in IMC is prob not "best practice".
From memory the almanac in the box is downloaded once a week, at least on the type I fly.
Possibly if the unit is only turned on once a week. Im sure its updated a little more often that that.