The engine was running and starting before, there is no reason why compression should fail so suddenly that the car isn't starting.
There is no harm in doing a compression test, but I don't think low compression is the cause for not starting. The time it took to start the engine don't sound unusual. Compression will decrease over time, because pistons and cylinders wear out, but not all of a sudden.
More likely there is a problem with the power supply to the glowers or the fuel supply. The electric lift pump isn't stock.
The glow plug indicator light isn't stock either, it was probably installed when the original glow plugs were installed. Originally there was a extra glow plug fitted on the dashboard behind the salt shaker, which started to glow red after 30 seconds or so. The upgrade to the newer glow plugs probably meant that a relay was installed with a indicator light. The fact that the light isn't glowing could mean the relay isn't working properly. Another reason might be a bad light or a bad connection to the light. You can try putting 12V directly to the glow plugs from the battery; do that for about 20 seconds and try starting the engine (an assistant is handy).
The fuel supply can be checked by opening the hard fuel lines to the injectors at the injectors. Just give it half a turn and try starting: diesel must squirt out. Do that for every cylinder.
Diesel in the cylinders isn't really a problem, you cannot flood it. Putting the accelerator pedal down during starting is not necessary and does not produce more fuel. When you pull on the gorilla knob during starting, you'll pull the lever on the pump in the starting position, that means that more than the normal maximum amount of fuel is injected. The position of the accelerator pedal doesn't change that.
The starter seems to be OK now, but if you want to clean it, you'll have to remove it from the car. The starter is connected to the engine with a couple of hex bolts and a bracket.
You might want to take a look at this short film, so you'll know how it works and what the parts are:
There are many descriptions of taking apart the starter on the big ol' internet.
The three screws of the solenoid are usually stuck. Proceed with care and WD40 here.
Remove the cap on the back, remove the locking ring on the axle
Unscrew the two long screws, visible in the picture below:
The bridge with the brushes looks like this:
Two of the brushes are connected to the starter housing, but you can remove them from the bridge if you remove the springs from the back. These springs are likely to spring away to an unknown destination, so take care.
The brushes should have some length, as is shown in the picture. Clean where the brushes connect to the rotating coil, shown on the left end in the picture below:
Use some fine oil when you put the thing together again. Don't oil the solenoid but clean and degrease it thoroughly.
Oil inside diesel engines will get dirty. You probably want to replace it. MB stated an intervat of 3000 miles, but that was with the 1970s oils. With modern oils you probably get away with 5000â€“7000 miles. It does not prevent starting, unless it is very thick.
You should not see oil when you remove the oil cap on top of the valve cover. Oil will have drained to the bottom, in the oil pan.