Well, for 1 and 2, that's more for you to prove that they can than for me to prove they can't since pre-existing lore shows that they can't, outside of certain more plot important npcs like the two you mentioned. For the most part, an npc has never been shown to cross class, except for a few early level situations where certain quest specific npcs have access to cure from conjurer. Outside of that, we have npcs with jobs that don't even have access to their cross classes, such as all the dragoons (someone correct me if I'm wrong on this). There are also npcs that have access to skills that go beyond what we can do, such as Alphinaud in the HW story. But there's no evidence of an npc being able to cross class outside of cure (but really, that itself is a different can of worms when you think about that one npc in the conjurer quest line who only wanted to use healing magick), excluding menders.

About a sentry being inside vs outside a dungeon, in some dungeons we really have no idea how deep inside the dungeon we actually are when we start. And with some dungeons, you have to remember that there are enemies within the first room, or viewing the first room from the hall, if there was a lone guard standing inside, more likely than not he would get attacked when his job is supposed to be preventing people from getting themselves hurt by going in.

Going back to Tataru and including Thancred, someone mentioned before that lore has already shown their growth into multi classing, while there is nothing showing a generic guards ability to do anything besides what their weapon shows. Also, as you pointed out with Tataru and being an Arcanist, npcs are not like the WoL, they can not just be any class or job without issue as they have limits to what they can do while protgaonist syndrome allows us to surpass those limits.

Overall, I don't care about the issue that much, I've tended to side with the people against it mainly cause I interpreted the people for it as being more hostile early on about this. I just don't agree with the statement that the lore can support them being in there and am usually against the idea of retconning lore to fit something in.