Replacing 1991 300SL water pump...
 

..how to remove the pump's lower front screw? Even with the head off it seems impossible without removing the PS pump and tensioner assembly! Does anyone out there have any tips ?

Right, tensioner assembly has to be removed and the power steering pump detached, although the lines can stay attached.

That is correct! Have done two now and it has to be done but is not as difficult as it may seem.

There's a Snap-On 13mm 12-point socket on a swivel + extension that lets you access all of the bolts without removing most of that stuff, it's the FLXM13. It's still easier if you remove the belt tensioner and the PS pump though - I have the tool but when I did the water pump on my 300E I ended up removing the pump anyways. It's pretty straightforward to remove the PS bolts, once you're in that far at least.