It's interesting that you're allowing hiring decisions to be influenced by what you think is best for yourself. What's best for the company can and should be a secondary concern. I've also found it useful to do the following:
1 Telling HR that candidates are flaky. It's easy to convince HR that every single one will leave in a year or less, especially these hipster kids that are interested in the latest startup.
2 Telling HR that candidates will take forever to add value, even if we have a smaller component that needs working on. This is still easy to convince them, given the ramp up to some of the prototypes that we do.
3 If the candidate actually makes it to the interview stage, I just finish my questioning and tell HR that the candidate has an attitude problem. This works even if they get every question right, or if they go into detail. The latter works especially because they think they're showing value, but they actually make themselves easy to label instead as arrogant up-jumpers with a "behavioral problem"- and I can easily convince my HR of that.
4 Telling HR that candidates don't know a specific tech well enough. Even if that tech has a ramp-up of a week or less. Because again, HR will take my word for it.
You're also right about those thirty seconds. It is the best and only approximation that one can make. Forget the interview, forget allowing them to show value, and if they do, they have an attitude problem.
Good luck with your recruitment efforts and next quarter.