1: I agree with Rice but also to add that it also probably helps to actually see them on either side. As if to say "ok person who isn't supposed to be here, we have fighter aircraft in either side of you. Lets not do anything stupid" sort of like a form of intimidation.
2

erhaps they're a little more lax on procedures when it comes to home turf. I mean when POTUS is out and about, I'm sure they're all wearing Kevlar but in the whitehouse, it's just sidearms. You do see at one point that they were grabbing assault rifles from weapon lockers and also I saw P90's, Uzi's and MP5's so they definitely have the hardware.
3: Again, Riceball got it right as he was still in the secret service so all his ID's worked, only he wasn't part of the POTUS security detail. It also wasn't that he couldn't be trusted, the president just saw him as a reminder and had him transferred. Not to mention all the safes he got into only had the presidents sat phone and a thumb print scanner. Dermis Mulroney was retired and working in the private sector so all his access would of been removed.