In switch position 2 why is the current through resistor Rin= -V_ /Rin+infinity?

There can't be.
The only current that can flow is any leakage current through the switch which is likely unmeasurable.

 

why is the current through resistor Rin= -V_ /Rin+infinity?

current = 0.
First I don' agree with the formula.
Second current=0.
Third, anything divided by infinity will be 0.