After spending so much time establishing how paranoid Hackman is, to the point that he seems barely able to open up to his longtime girlfriend/acquaintance, letting all those people into his office and then letting the other woman he barely knew stay there (allowing her to steal the tapes) just doesn't come across as plausible to me.
Well, they had to go somewhere
. It would have been less plausible if he took them to his apartment, even though that would seem a better place for a party. It's not as if he lets his competitor in on his trade secrets; he stops the tape when Stan begins to play it and keeps his best equipment behind a locked gate. He does explain his recent operation, but only to show up his competitor, who he doesn't seem to like at all.
As for the hooker, Harry is paranoid, of course, but it seems to me that his overriding emotion in the film is guilt, not paranoia. To him, she's a surrogate for Amy; he feels guilty about never opening up to Amy now that she's vanished. He's also distracted by his guilt about causing another murder; in a strange way the hooker is also a surrogate for the women he thinks is going to be killed.
Maybe these are just excuses, but it seems to me that as boastful as he is about not being personally involved ("I don't care what they're talking about -- I just want a big fat recording."), Harry Caul constantly feels guilty about what he does.