No baffle under oil cap------1 & 3 get oil fouled-----maybe valve seals/guides bad and your pouring oil directly into cylinders??
I agree that low compression on 2 adjaction cylinders sometimes indicates leaking head gasket, but also on engines with the intake having 2 ports side by side that can also mean(especially on aftermarket) bad seal around those two ports plus in this case it also is running lean/whiscling?

Leaky solenoids/fittings /plates on nitous systems is quite common and is probably the only way you could kill the milage this much-unless he is mixing mpg(miles per gallon ) with Kpl(kilo per liter)

The header to head is just one of several places for an exhaust leak-------just think about how many joints are fabbed and welded and then installed with ill fitting expansion joints, flex joints, etc-