Well that was fun. In the pouring rain first thing this morning, I got that o-ring outta there, so that's all good. *Given the grief that this causes to the uninitiated, I respectfully suggest that the first post be edited to include looking out for the o-ring that might be stuck on the engine, and that it must be removed.
I started the engine, and no leaks of course, but that roughness at higher RPMs was still present. I shut it off, unclipped and re-connected the two plugs on the MAF, and checked that all the plugs on the computer were tight. No change. Again, just stepping on the gas pedal lightly to bring the RPMs up a tiny bit sounds & feels normal, but when I more like "stomp on it" so to speak, it becomes rough and won't climb in RPMs like it should. The air smelled rich with fuel when I stepped out of the van.
Prior to all this, besides giving the entire engine compartment a thorough clean - including taking off the plastic cowlings below the wipers - the only other thing I did the other day was remove the top of the fuse box and unbolt it from the firewall in order to get that battery tray out.
I'm stumped.