Home Sec is not arty farty leftie - it's clearly a right wing government, he's supposed to be a straight-laced Tory sort.
The reason the gun was left but just the bullets changed would be because they did not want him to suspect anything if he checked on the gun. But why he didn't check the chamber and notice they were blanks is anyone's guess - trauma I suppose.
But, yes WHO changed the bullets, this is the key question. Though 'keys' are not the 'key' to it, because most people involved here (SS, high level police/counter terrorism etc) could get in without a key.
I thought we did see him put the gun in the panelling, obvs at some point he moved it (after all that hard work redoing it, had to do it again) and the reason Louise looked perplexed was not because she knew it should be there but because it looked such a likely hiding place (perhaps he left it looking slightly 'new' on purpose).
Why would they not have found it in the light fitting though, it's hardly an unusual hiding place.
Not sure about the memory stick either, I didn't see him use one with the tablet and most tablets don't have USB ports now - wonder if that was a plant and he doesn't know it is there, they made a show of him feeling around and not touching it - if he knew it was there he would have gone back to check on it once he knew the bullet was blank.