I have a questions:

1) Has the ignition system been checked?

As already stated, your symptoms could be from leakage from a wire(s) or the coil could be going bad. I would call IPD and get an ignition tune up kit, plugs and wires, cap and rotor and replace it all. That may have an effect, or the coil may need to be replaced too. It's not too expensive, call around the USA for it. All of the above are DIY repairs.

You have to be careful not to allow oil or water to run past the spark plug cover and into the wells holding the plugs. Water or oil is conductive, so electricity could leak out of an old wire and ground to the engine, effectively causing a miss, and chugging perforance.

The fuel pump is inside the fuel tank, so it would require dropping it to remove. You could get a fuel pressure tester or have it checked at the injector manifold before dropping the tank.

I would check the ignition system first and do the tune up if it hasn't been done. If it has, then get the fuel pressure checked.

You didn't ruin your engine.
Manufacturer's consider a quart of oil every 800 miles to be acceptable.
If the oil isn't on the outside of the engine, or on the ground, it has to been burning. Could be any number of things, including valve seals.
Now, you are using a quart of oil every 1500 miles. Assuming you drive 15,000 miles a year, you would be adding about 10 quarts. At $5 a quart, that is $50.
It would probably cost you more in labor fees to have a technician check out your car to see if he/she could find where the oil is going.

Now, the performance is another thing.

It sounds like you have one of the models that has given the 850 a bad name for maintenance expenses. We ('94 850, 140k miles) have been luckier, with only two significant problems: the heated seats and no hot air to the feet (the dealer did not solve this one -shame on them - which turned out to be a bad thermostat.

Our Volvo now consumes about a quart/2k, and since my wife is the primary driver, I have to be vigilant.

I presume you have replaced the fuel filter (located next to the right rear wheel at 60k per spec? If not that might account for the behavior. The no-load behavior suggests this to me, since not much fuel would be demanded then.

On other cars, bad spark plug wires have accounted for similar behavior.

I am not a mechanical expert, but have been around for more than a few years. Hope this helps.