Sad story. They usually fail when the thrust piece has been so worn away that the gears don't properly mesh. At first the gear face is riding on a brass bearing, but that wears away, and all that is left is aluminum.
You can check the thrust piece by removing it- there is a retaining bolt on the side of the block that comes out that just holds onto a groove in the thrust piece. However, on my car, the head prevented it from coming out- I had to grind away a bit before the thrust piece would pull up. Never expected the interference! Clean the top area, pull it out and have a look.
But the drive shaft running form the injection pump to timing device- I would find it hard to believe that could have an effect on the timing, because the gear drives the oil pump. If the gear was damaged, your oil pump wouldn't be driving properly. The shaft running to the timing device should remain intact even if the gear was damaged.
The "plain bearing" at the front can wear, but this shouldn't effect timing either as it is an issue with fore and aft play in the shaft/timing device. About the time you are eating vacuum pumps on a regular basis you can be suspect of this because they do wear out.
I suppose if someone tried to start the engine with a huge pipe wrench on the timing device, the splines on the IP would get destroyed, but I cannot see otherwise how this could get damaged.
When I had mine out, the shaft did want to rotate out of position. It is possible the IP isn't timed correctly. This can be corrected by re-inserting the IP, with all respective parts in the proper place; just handle that IP like it's full of nitroglycerine (very carefully!!!) stick it in and hopefully it hasn't sprung out of position. While you are in there, check the thrust piece and plain shaft for wear, and replace them if they are out of spec! Tear down the timing device and inspect- there are two springs inside that can wear. Take pics as you go. Mercedes still lists those springs at around $6.00 US apeice, so it's not TOO costly. I found two new, old stock thrust pieces on Ebay for less than 1/2 the price of a new one.

I did these things on my 240D along with a new timing chain. The results are nothing short of amazing. I had four of us in the car today at 72MPH on the freeway and there was barely any engine sound. They do like to go! The car was much quieter than on the highway at 55-60.
I have said this before- for a long time I thought it would have taken crazy people with the money to buy a smoky, stinky, noisy diesel for around $30,000 in 1979. Now that nearly all the suspension has been renewed, and some important engine parts have been replaced, I need no further encouragement to continue. I know do know why those folks bought these cars!

Cheers,

snapped_bolt

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'81 240D For now, a good place to borrow new parts
'80 300TD Probably will be put back into service!
'79 240D BACK IN SERVICE SINCE 09/16; limited use, oil leak. Guide pin r/sealed/replaced. Still a leak. Front crank seal....