I'm still a little worried about effectively fighting pumps though with a 3.5 day delay in the conversion time. I could easily see converters getting their HBD right about the time the price pump ends

I don't think it's that big a problem. People who already hold HBD can sell it immediately and initiate a conversion to get it back (with an expected profit) in three days. If there were a lending market, you could even borrow HBD, sell it, and then use the conversion proceeds to pay back the loan (with some risk). If this happens regularly it makes holding even more HBD and waiting for such a pump to be more profitable, meaning more people will do it.

We could also just reduce the 3.5 days in both directions to say 24 hours, or even as you say 8 hours. It's basically the same argument that was made when reducing it from 7 days to 3.5 days: reduced defense against manipulation (which maybe is currently overkill still even at 3.5 days) in exchange for more responsiveness.

But we haven't really had a problem with HBD price dumps (that aren't so large as to involve the haircut) taking 3.5 days to correct. People are pretty willing to tie up capital and take a bit of price risk for 3.5 days. I don't think we necessarily need to go from having no HIVE->HBD conversion mechanism at all to assuming that a slow one won't work. It can only make matters better even if not perfect, and if it turns out not to be sufficient we can do more.