Update #1:
Appreciate all of the information, opinions, and suggestions. As of now, I will not be dropping the pan, changing the oil, or replacing the filter. The reasoning is that I don't want to trigger something which ends up ruining the transmission. I'll just let it be until the inevitable and go from there.
As for plugs/coil packs, I replaced the coil packs - no improvement. Since I didn't want to risk myself breaking the SP515 plugs, I took the new SP546 plugs to a local shop and they should have them replaced this Monday. I took it in this morning, but they couldn't get the plugs off initially, so they're soaking the plugs over the weekend. Will give another update in a couple days.
Also, I forgot to mention this in the initial post: I had the catalytic converters replaced a while ago and the mechanic made a poor connection from the new converter to the manifold, which has resulted over time into pretty loud and smelly exhaust leak - sounds like a go-kart. Is there a possibility that this exhaust leak is also causing the engine shuttering, etc. symptoms?