This job is a pain.
To get access did you remove the front bumper, radiator( unbolt lift and tilt forward) set aside the AC and the Power steering, remove the idler and air filter housing. This might get enough access to get to that rear bolt. Mine had been replaced once before and someone used a conventional bolt on that one location.
Mine is a 1991 190E 2.6 and may be different, but that's what it took to get the pump off.
And mine only had a bad bypass hose. I also changed the pump,idler pully and bearing, fan pully and bearing assembly,sepentine belt and all hoses. Don't want to go back there again.
I feel your pain, Jeff

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Jeff

97 E320, ours 217,000miles
91 190E 2.6 daily driver 242,000miles
74 MGB,mine 120,000?
2006 BE350/Proline 21, 50 hours