It's probably a combination of all the above. But without smoke, it seems to me to be a leakage problem.
An "outside" leak would show up on the engine, trans, firewall, crossmember, etc. We've all seen what a mess even a small amount of oil can make.
A few quarts of oil in the cooling system...hmm...I would think there would be oil floating in the radiator, unless it's being spewed out the overflow.
A few quarts of oil in the exhaust system might not show up anywhere until the low spots are filled: muffler, dips in the piping, etc.
It's possible it's burning some without making much smoke, and the compression numbers are not encouraging. A few quarts through the cylinders would probably show up on the plugs, though, so...
There's some incriminating evidence suggesting some thorough investigation is required. I hope it's something easy and inexpensive to fix. If a rebuild is necessary, it's a good time to hop it up a bit!
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