DC, any progress? I can't help but think that getting the rest of the water out will require, as you know, a trash pump and plenty of muddy trips into the hole for proper placement of the suction line. Once the lowest spot reveals itself, you may try digging a pit there, lining it with a half barrel to keep the pit open. The barrel would likely need to be peppered with drilled/cut holes and a weed barrier mesh wrapped around its outside to keep the sludge from filling the barrel, but still allow water to get in. I have not done this by no means, but its the first thing I would try if I was not able to get the big boys in to scoop it out wet. All I really know is that muck is very sloppy, a lot like quick sand I'd imagine, very hard to shove and control.
Your pond does not look to be too big, there are medium to large bucket hoes that could make the reach and scoop out the soupy muck which may be the only way if you can not keep water from running into the pond. You will need to eliminate any springs or gullies from feeding the hole or your muck will remain wet. The muck that was pushed over my dam last October still pumped up and down as I drove my 8N tractor over it in early April. We had a dry winter, but the pumping muck was about 10 feet deep on the back side of the dam. The stuff hold water like it was meant too.
