I guess the issue is I was assuming that witnesses will publish any adverse price move during the last hour (including at the current time). In such as case, I don't believe it can be gamed.

If they aren't doing that (ie. if they were only posting on some time basis without publishing faster when there is price change of say more than a few percent), using the worst case price in an hour would fail (since we wouldn't really have the worst case price). However, I still don't see how it would be gamed if we are correctly getting the worst case price during the past hour. But if your argument is that we can't count on that happening, that's a different situation.

I think there's a tradeoff here: having the conversion take 3.5 days means it's won't be possible to stop short price rallys in HBD. But instant conversions do increase the possibility of gaming, if witnesses aren't able to provide fast enough price change data.

Another option is to compromise at some intermediate point, for example setting the conversion delay to something like a couple of hours, and keeping the worst case price check during the hour prior to the operation plus some additional small period of time after the conversion to ensure the current price would likely be included in the "worst price during hour" calculation. EDIT: after thinking about it, I guess we can't reasonably use any pricing after the operation is requested, because then that could potentially be used to attack converters.