Hi,
To clarify: I see no improvement in using a bridge configuration. Simple 4 wire measurement should do the job.
1)
I can not give a value, because it depends on your "substance". We do not know anything about your "substance".
From the signal - or better say the signal-to-noise - view: The higher the current, the better.
2)
Yes. Don´t expect a signal improvement. It´s just to reduce power consumption.
3)
Yes.
Expensive solution. You can build your own lock in amplifier with cheap CMOS analog switches and a comparator.
Klaus