Assuming a non-basic economy fare:
1: Yes, as long as the fare rules allow them to be combined. Usually this is the case, but sometimes not.
2: Reading the fare rules. In reality, you'll have to just price it out and see. Taxes on the return will likely be higher, but the fares should be roughly the same for a round trip, assuming same fare bucket.
3: I don't know how you cancel a leg after flying it. If you mean cancel leg #2 after flying #1, it will reprice #1 as a one-way flight, which likely is the same (or more than) the round trip, so it basically results in forfeiting the value.
thank you! Yes that is what I meant.