Sorry to hear about this bad luck, and here's hoping your current boss and contacts help you to find another place soon.
The second and third questions here give some of the answers you need.
http://www.immigration.govt.nz/migra...rk/default.htm
I don't know about your last query - I think you'd have to ask INZ about that. But there's not only the possibility of tweaking your existing work visa, as you could apply for an entirely new visa if you find a new job that entitles you.
In fact, in your place, I'd talk your whole situation over with INZ so you are SURE of your options. We on the forum are mainly members of the public whose knowledge comes from sharing what has happened (so there's a lot of past experience on the threads), and what is written in INZ rules. However, this is MAINLY about people's arrival in NZ - the first visa. A lot of people don't come back to the forum once they're settling into NZ life. I don't remember anyone talking us through what happened with a visa change like you're hoping to be able to do.
I do know from the regulations that, once the job named on your visa comes to an end, the visa also stops. INZ usually allows a person a bit of time to try to sort out their situation, but in principle, no visa means having to leave the country. If they regard your WHV as still running although you got the work visa on top of that, you may have some extra time, but that's something you should check with them.