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The first thing I noticed is that all your circuits use separate resistors to convert the current of each side to a voltage.  That will cause a differential mode error signal to the extent the resistors aren't equal.

I would start with a <i>single resistor</i> for the current to flow thru to convert the current to a voltage.  After that, you convert the now differential voltage to a single-ended voltage in the normal way.  An off the shelf "instrumentation amp" would be a good start.  I wouldn't try to create my own diff amp unless the off the shelf integrated solutions were found to be inadequate for some reason.  It will be very hard to beat the common mode reject of an integrated and factory trimmed instrumentation amp.