Since you don't seem to have a buffer in front according to your "schematic", that might be cause of your troubles.
Series resistance pots for the mixing as you've drawn it up isn't great.
On phase cancellation: as both the muff fuzz and LPB-1 invert the phase, there shouldn't be any issue give you've done anything right.
Impedances shouldn't be an issue either with an input buffer to split.
There's no need for 2 seperate buffers for signal splitting either.
Take some "inspiration" from the clean blend circuit here:
The only issue I see is that the muff fuzz might not like being placed after a buffer and will be painfully bright. In that case you can put a cap like 470p or so between the base and collector of its first transistor.