Car won't start, suspecting there is an o-ring leak, I need more input.

[Bowzah]

The body of this post is copied off of a reddit post, so it might have some reddit formatting that I may have misssed

I'm 19, this is my first ever car, I got it as a gift. I know barely anything about cars and I'm learning as I go.

To elaborate a bit:

First I had a rattling noise coming from my engine bay, went to a mechanic, they said it was the alternator, which we changed and the rattling was fixed. Also changed fuel filter and mechanic also changed the [manual hand pump](https://i.postimg.cc/wTs2Dqdz/Screen...48-Gallery.jpg) (circled with red in photo, he said it had started to give out).

Fast forward about a week and car fails to start.
Got a jump in case it's the battery and that failed. Called our mechanic again, he came and started the car and put that inline filter thingy [(circled with blue)](https://i.postimg.cc/wTs2Dqdz/Screen...48-Gallery.jpg)

Car was fine the day we got it back and the next day.

Then I go to drive my car and it fails to start again. My mechanic kept nagging me that it's the battery so I tried to jump start the car to no avail, surprise surprise.

Got fed up with the obvious avoidance of my old mechanic so I found a different one. He came, I explained the problem. He proceeded to pump constantly on the little pump as I was turning the key and with my foot on the gas pedal and we started the car. He took another look around the engine bay and noticed the dark color of the diesel in the filter thingy and noticed it was black. That immediately prompted him to suspect (and not sure about the name in english) that the radial shaft seals/oil seals in the engine are giving out, and leaking oil into the fuel and causing it to turn black. And also causing air to leak into fuel lines, thereby causing all the pressure to drop and the fuel to fully return to the reservoir, to the point that it'd need lots of manual pumping to get enough oil back into the fuel lines and fuel pump.

The first mechanic lost me a good 3 months by fixing and "fixing" my car, and never ever suggested anything like this. It became apparent after some time that he's full of sh*t because he kept avoiding fixing or attempting to diagnose the real cause of the car's failing to start.

My question is, does this second mechanic's opinion make sense or is he also trying to scam me into paying unnecessary money.