I understand that Glyphosate can be used for killing stumps.  Holes are drilled in the cut surface and neat Glyphosate poured into the holes.  Of course it will take some time and there may not be much point unless you're prepared to wait for the stump to rot out.  All you're doing is preventing it from re-growing.  Depending on the scale of the problem it's probably just as effective to find someone with a mini digger to lift them.
http://www.diylife.com/2008/05/30/avant-yard-kill-a-tree-with-herbicide/
I don't know if Mayor sell pre-mixed concrete.  Probably, as they sell most things, however, it's probably going to work out much more expensive than mixing your own depending on how much you have to do.
Don't know whether the dry canal is on or off.