I searched, but found no relevant entries.

My '04 Quest with 3.5 V6 had run 160k miles on the ORIGINAL plugs, and for the last 20k miles or so I would have an intermittent misfire that would happen at idle or low speed, but would disappear at higher speed. This told me the plugs were due for a change, because the lower ignition energy at idle was causing the misfire with the worn (higher-gap) plugs.

So I FINALLY did the plug-swap myself, which was a MAJOR hassle, and replaced the original double-platinum NGK plugs with regular platinum plugs Bought them from Rock Auto).

Plug misfire is WORSE now, and the "service engine soon" light is almost always on or flashing. Code reader said P1800 AND P0300 (RANDOM MISFIRE). However, I can start it up in the morning and drive to work and it will run PERFECT, but if I shut it off and THEN fire it back up, it will run like crap.

Could it be that I didn't use the OEM double-platinum plugs? Should I go back in and replace the coil packs (wish I'd done that the first time)?

From my experience with half a dozen different cars, misfire is very hard to cause with spark plugs.
They just seemed to work for me, the worst case being when I used Bosch platinum plugs in Maxima's VG30 (big mistake!) and their central electrode burned into the insulator, but it would still not misfire, just the gas mileage sucked. Look at the EGR system first.