Since only electron's flow in electricity and electrons have negative charge, then why we don't say —1amps (—1C/s)?
Secondly, as conventional way we write down independent variable in $x$ axis and dependent on $y$ axis then $I/V$ ($V$ on $x$ and $I$ on $y$ axis) should not indicate conductance?
If conductance is constant then would the resistance also be constant?
1) The convention of electron charge sign does not make any real difference. If you speak of current magnitude you use positive numbers. If you have some oriented device, e.g. battery, you may surely write "+1 A" for discharging and "-1 A" for charging etc.
2) In fact in nonlinear systems (e.g. tunnel diodes) the IV curve can become quite complicated. It is better to look at it as a set of all points of voltage and current that may be ever measured at the device. And yes, I/V is conductance, but its use makes sense to me only in linear systems.
3) Yes, conductance is 1/resistance. If one is constant, the second is as well.