As you discovered those glow plugs go through the rocker box and are on the low side, where oil puddles and sits. Pulling the plugs is pulling them through that oil. There is a tool made for pulling that harness but usually replacing the harness is something you can count on doing , and pulling the fender liners and tires makes it much easier for both sides and is worth the extra effort. Low compression an a cylinder? A quick trick is a squirt of oil in that piston bore will wet the rings and tell you if it's rings sealing poorly as it would bring that compression number way up. You mentioned blow-by but I never saw it being shown. If you pull the oil cap while the engine is hot and at idle and it sprays and smokes like a train out the filler neck then you need rings. If this truck was in my hands it would be a set of piston rings, hit lightly with a hone to break the wall glaze and head gaskets, dummy plugs and stand pipes, an oil cooler and a Snap-To-Connect fitting update on High Pressure Oil Pump and ARP studs to replace the head bolts. These aren't super expensive for what you get when you are done, just labor and time intensive to get installed. Since it sat there's a good chance the turbo unison ring is also rusted stuck. Everything can be done in-cab by experienced people who have worked on these trucks before. Just because it's possible doesn't mean it should be done. Smart man would take the engine out and work on it without wrecking his body crawling around on the ground or a floor. Don't remove the camshaft or crank for any of this work unless there is evidence of bearing damage on the rods. The fact these engines can't run without oil pressure means you almost never have oil starvation problems that take out those major bearings. They are good for 350-600K miles. Doing things to add power to these trucks is not really needed. They need good fuel pressure and the above changes and a ECU Tune and you'll get 425HP out of them at the tires. Asking for more is a waste of your money. Bigger injectors or bigger turbo won't help because of how the camshaft is designed for emissions. Replacing the camshaft won't help because the breathing of the engine is limited by the exhaust valves. They are small and flow poorly. Dollar for dollar the biggest power adder is the Tune and everything else is dumping money for little gain. If you add more power with a tune then you will want to replace two solenoids in the automatic transmission to keep your clutches from being burnt before they fully lock. Hundreds of forum posts out there telling you what two solenoids and how to do it and why it's done. It's a learning experience and one you could spend 3000$ on the low side learning.

It’s normal for glow plug to have oil on them when you pull them