OK, so a few different questions here:
Can I make a 2 week reservation and call in between to have them bill me separately?
No you cannot.
Or make 2 one week reservation and instead of having to bring the car in I'd just phone in and tell them to close out the previous week and transfer it to the new week without having to bring the car in?
Again, no.
I wouldn't mind returning the car to get another murano with less miles (current one at 30k and was suppose to get a oil change at 15k miles lol) but I figured I didn't want to risk having to return the car to only get a car I didn't want since I've tried getting a murano for a year now with it always being unavailable.
While I think hoping for another Murano with less mileage is a bit too much to ask for, I think you have a very good chance of bringing back the rental for week 1 and just asking them to close out the contract for that and then re-assigning you the same vehicle. If they're really feeling generous, they might even agree to send it through the wash if you want to wait for a bit, but that may be pushing it a bit.
Note that this cannot be done over the phone. You must physically return to the renting location to close out the rental for week 1 and check out the rental for week 2.
Also note that you need to make sure you return the car after week 1 with a full tank of gas even if you expect to leave with the same vehicle. They need to record the actual mileage and fuel level when they close out the first contract, so if you return with less than a full tank you will be charged (unless of course you prepaid for fuel).