It's finished
 

After 17 months and 5 major repairs my 2012 R350 Bluetec has finally bit the dust. At the end of March I saw a massive oil puddle and then got the red screen of doom, dealer says the rear main seal is gone.
They fixed it 6 hours later - huge puddle again - car has to be towed back
3 days later it's fixed again - 4 days later - huge puddle of oil, red screen of doom - car gets towed back
This time they say "oh we're going with the steel option rather than aluminium this time" They let it sit for 24 hours, no leaks everyone is pleased. They take it for a test drive and it dies. Cannot be restarted, they order a new starter and still no go.

Result given to me yesterday - Engine is seized - $10,000 plus labour.

I am livid, and not mechanic enough to know if their failures to repair the rear main has led to this. Appreciate any tips or advise you have.

Sounds like you need a respected third party mechanic to evaluate what's really going on and an attorney...

This.