EGR valve stuck open will stay stuck open regardless of vacuum line disconnected. I've had a stuck open EGR valve, and it produces the exact symptoms you describe (low power, smokey exhaust). It doesn't NOT contribute to the blow by.

Fashion a plate to block off the exhaust feed into the EGR (use steel not aluminum).

Keep marching through the other items mentioned, I'll bet the engine is basically sound but may have high oil consumption. Oil is cheaper and easier than a replacement engine.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC