Wow, I read this earlier today and noticed no one has offered any help. I recognize the feeling you are experiencing and truly feel for you. I know that doesn't do you much good so I got my manual out for my old 1975 240D which covers the 615, 616 and 617 engines to look at the situation. While I do not want to make light of the problem, I do not think it looks all bad.
First, one end of the chain is most likely draped over the intermediate sprocket which serves as the injection timer, which a a sprocket about the size of the camshaft sprocket, but located in the bulge on the driver's side of the engine block, with the top of the sprocket and likely that end of the chain just below the head parting line. This area is being obscured from your view and therefore your access to get the chain by the inner and outer slide rails that guide the chain as it comes off the camshaft gear (sprocket) on the driver's side of the engine at the top of the head (if the vale cover is off you can see these items as they stick up nearly half way up the camshaft sprocket). I believe with this assembly removed you will find one end of the chain. My manual suggests this is relatively simple, you loosen the screw that is behind the arm connecting the inner slide rail to the outer one and it should slide forward enough to be removed.
Anyway, once you get an unobstructed view to see if the chain is actually slumped over the intermediate sprocket, you have to be very careful not to disturb things and let the chain just fall all the way into the oil pan. If you can reach the chain and avoid letting it fall further you may have saved some further disassembly to get to the next set of slide rails just before the chain gets to the crankshaft sprocket on the driver's side. At this point if you are lucky (and sadly you don't sound lucky) you may also be able to recover the other end, as it may be piled up on top of the crankshaft gear. With the driver's side end secure you can go after the other end. Be careful as I think the chain may just slide all the way down to the oil pan and if that happens you have to do lots of disassembly to thread the chain back into its path.
To get the other end of the chain if it is visible but out of reach you will need to remove the chain tensioner and likely the big tensioning rail on the passenger side of the engine. This is the baby with the broken bearing pin, I believe. To do this job you must have the radiator out and the cranshaft balancing wheel and belt pulley off. Personally I would buy a new pin, and then drill out the old one enough to tap a thread into what is left of it and use a puller to get it out. If that fails again, I think you have to drill it out and then deal with the debris inside the engine by flushing and removing the oil pan.
If you have to go through the oil pan to get to the passenger side end of the chain, you can drop a wire or the like down and tie it to the end of the chain and pull it up. That side of the chain path is simple, no two sided guides or anything, just a straight shot up past the tensioning rail to the camshaft sprocket.
If the chain is in the oil pan, I would take the car to a shop with Mercedes experience and have them finish the job. There are special tools and the like required to reassemble items that have to be timed and so on to make it work right.
I hope this helps and would be glad to help if you have questions I can get answers from in this manual. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)