Assuming you have dual inputs on your PC you need a splitter cable that takes the TRRS connector from the iPhone headset to separate TRS headphone and TS mic connectors. http://en.wikipedia.org/wiki/TRS_connector
I have one, but it came as part of a kit with a universal headset.
Yes. Read the Wiki article I linked. You need to move the mic connection which is back between the last ring and sleeve TR(RS) to the tip and sleeve of a mono TS connector. You can build your own cable that isn't a splitter if you want to.