Engine past recommended oil change?

Not likely. Broken valves are extremely uncommon, even on vehicles that have regularly ignored maintenance. As an experiment for my students, I had an engine in an '88 Grand Caravan that I thought was about to fail, so I stopped changing the oil. It went 14 years without an oil change, (roughly 100,000 miles). It did need a quart about every 1,000 miles, and I changed the filter every other year. I only stopped driving it because it got so rusty, the carpet was the only thing holding the front and rear together.

Most newer engines use some type of variable valve timing. Part of the control system uses engine oil under pressure, and control valves with rather small orifices. For those systems to work properly, it is more important than before to keep the oil clean. Missing one oil change isn't likely to cause problems. Missing oil changes often will eventually lead to increased engine wear, but not necessarily catastrophic failures.

Also be aware a lot of manufacturers have unusually high mileage recommendations for oil changes to make their cost of maintenance appear to be lower than that of their competitors. For both of my vehicles, the recommendation is 10,000 miles. I'm more comfortable changing oil at 5,000 miles.

Depending on how the valve broke, it could cause low compression in one cylinder, not two. Removing the intake manifold doesn't have anything to do with compression in the cylinders. It simply directs the flow of fresh air into the cylinders.

If the two cylinders with low compression are right next to each other, that is the classic symptom of a leaking cylinder head gasket. There were three engines available for your model. Two are 4-cylinder engines, and one of those uses a turbocharger. Those pump in extra air to get more power from a little engine. The cylinders have to work harder and that adds a little extra pressure to the cylinder head gasket. Regardless, at the mileage you listed, head gasket failures are not uncommon. I'd suspect that long before a broken valve.

Lack of one oil change doesn't concern me. What concerns me more is whether the coolant was replaced at the recommended intervals. Antifreeze is alcohol, and will always be alcohol, but it's the additives that wear out. That used to be in two years, but today some products last longer. One of those additives is corrosion inhibitors, mainly to protect head gaskets, heater cores, and radiators. Even when the coolant is replaced regularly, head gasket failures still occur, often at much lower mileages that what you have.

Whether it's a broken valve or a leaking head gasket, the cylinder head has to be removed to do the repairs. There's other parts that can fail and mimic a broken valve. Those will cause lower compression in one cylinder, but the fix is usually easier. Another thing that tends to point away from a valve problem is the most common way one would break would leave part of it rattling around in the cylinder, and that would result in serious damage, often not repairable. If you were able to drive the vehicle with simply a misfire, I'm skeptical of a broken valve until I see a photo of it.

The place where I've seen a missed oil change come into play is when a dealer is looking for any reason to deny a repair under warranty. Whatever missed scheduled maintenance they can find will become the cause of the failure, so, "claim denied".