Speaking of theories - here's a "no foul play" version that rather fits the few observable facts.
The facts we can easily observe (Yes, there are certainly others - but we don't have access to them)
Fact: It was an unusually cold night in November in Oklahoma.
Fact: Three teens provided a somewhat shady story to parents about "going to football game"
Fact: Somehow they actually ended up down on a lonely road by a somewhat distant lake far from the football game.
Fact: The car shows no front end damage - it didn't hit the water at speed - but did end up in the water.
Fact: The windows are obviously up. No sign they never tried to get out.
O.K. so now the THEORY part. So, three very slightly naughty teens who are supposed to be at a football game instead take a little unauthorized joy ride in their friend's cool new 69 Camaro. They end up sitting in the cool new car on a lonely isolated boat ramp road down by the lake. Maybe they're listening to the radio, maybe they're talking, maybe even drinking a 3.2 Oklahoma beer they somehow got hold of? whatever - Sitting there, doing whatever they were doing, but it's cold...28 degrees! Driver starts the engine to run the heater because it was so cold...and the windows are rolled up.
Result? A little carbon monoxide leaks in from somewhere. (yes, 1000 ppm concentrations can render a person unconscious in less than fifteen minutes, and fuzzy headed in five) So, one by one, they get fuzzy headed, confused, maybe near to passing out or even passed out? Either passed out from CO, or perhaps in some very confused state, or maybe even dropping off to unconsciousness or with a last desperate jerk of a semi-conscious hand, the teen behind the wheel blurrily realizes something is badly wrong...tries to do something and clicks the car into neutral. (or he simply passes out and lets his foot off the brake, if the car is a 4 speed and he's been "holding" it there in neutral with the engine running and a foot on the brake...whatever) Either way, that's it. The car rolls straight on into the lake, (slowly) sinking and continuing to roll right down the submerged road as it slowly sinks into oblivion carrying the unconscious trio to their untimely deaths. Fortunately, they are already oblivious to their fate, thanks to the carbon monoxide poisoning.
Ockham's razor. The simplest explanation consistent with observable phenomenon is probably the truth.
I'm no cop, but my questions when looking at the car would be;
1. 4 speed or automatic car?
2. position of gear shift selector (drive or neutral if automatic, or other? Neutral or WHAT gear, if a 4 speed?)
3. position of key in ignition (running or not?)
4. emergency brake set or partially set or not set?
5. position of heater control levers? Blower switch?
6. headlight switch position? on, parking lamps, off?
7. Also curious - was this a power window car? (some 69 Camaros did have them)
8. What direction would the car have been pointing on the road relative to the area's prevailing wind direction? (a very slight tailwind could easily push the CO from the exhaust gasses right into the car through some relatively small opening, like a missed body seam sealer, a slightly open rear window, etc.)
Observing those things might give additional useful evidence to the state of the car in its final moments before submerging.