Did anyone else find it increasingly implausible that the teachers kept trying to speak to Harry while he was thinking? The first one or even two approaches made sense, but if a person who has just lost a close friend says that they want to be alone until dinner, the only sensible course of action seems to be to say "okay" and leave them alone.

Eh. In more typical situations it isn't that odd to force people who want to be alone in bleak times to not be alone. (I'm not sure if the primary impetus here is an anti-suicide measure, the thought that it improves mood, despite the annoyance, or the inability of people who want to help to convince themselves they are helping without visible action.) That Quirrel recommended it makes it more sensible (though, as the strong reader suspicion goes, Quirrel is trying to sabotage their relationship with Harry), even though Harry protests.