1998 Spacia Misfire - Broken Waste-Spark Coil?

[jmcc]

Hi, I'm hoping someone might be able to help me diagnose my ignition issue.

I bought my van a couple of months ago, no major issues so far, until I spent last weekend driving ~600km including ~40km of unsealed corrugations. On the way home, while I'm driving on the highway again, the engine loses a bit of power and I can't get above 90km/h. First thing I checked was the air filter, pretty dusty after the weekend so I banged it out and tried again. Still no better so I try taking it out completely, still no improvement.

Once I got home, I was able to check the plugs, ended up replacing them all while I was at it. Cylinder 2 looked like it had been misfiring and sure enough, no spark in that plug. Pulling off that spark lead while the engine was running made no difference, while when pulled off any of the others the engine faltered a bit. Looks like a waste-spark ignition, so I switched the leads coming out of the coil, the problem cylinder moves to 3, so that seems to confirm that the spark plugs, boots and leads are ok and the problem is the high-voltage output for plug 3 on the coil. 

What I'm confused about is if plugs 2 and 3 are meant to be in series for a waste-spark ignition, how can 2 still work if 3 isn't sparking? Is it that there are actually two secondary coils, one for each plug? If that's the case, is it possible that one of those coils has failed, maybe because of the corrugations?

It seems like the 2,3 coil unit needs to be replaced, but I'd like to be sure because the part costs $200+. Are there any resistances that I should be checking to test the connections?

Thanks for any advice!