Thanks for all the input! After a few more tries, with every potion/scroll/flask buff I could afford, the best I could do was wiping at 20-25%. I've concluded I just need more gear/levels, or a 5th account, to have the necessary margin to beat these encounters.

The good news is I earned enough gold to get flying mounts for everyone, finally giving me access to Tempest Keep. And boy, after wiping in Hellfire I'm cutting through these instances like a hot knife through butter (on normal mode), and there are a bunch of nice gear upgrades to look forward to here.