Welcome Shoboshi.
Here is an old post regarding "Joint Registration"
Wifey and I are already in the process of JOINT REGISTRATION for all the firearms in our possession.
I am joint registering my pistols and rifles to her and vice-a-versa.

In our case, WE needed to bring OUR affadavits of firearms training, registration forms for all fire arms previously registered to be joint registered State I.D.'s and our MARRIAGE LICENSE.
If you are also joint registering rifle's, transferee will need current Long arm's acquisition forms (or get them there on you're first trip as you're gonna wait the 14 days any how

).
FYI
Last time I went in they asked if I had a permit to acquire Long arms.
I said yes, but it was expired. They told me I needed to return the expired form.

Looked it up and could find no evidence of such requirement UNTIL I LOOKED AT THE BOTTOM OF MY PERMIT TO ACQUIRE LONG ARMS.
Yup, there it is right on the bottom of the form.
We sat on the Group W bench and filled out all the forms we have numurously filled out in the past (Medical, Medical Marijuane (NEW

), Mental Health, permit to acquire, Long Arm acqusition and then filled out the permit to acquire form for each of each others firearms.
Whew
I had brought all of our firearms the first time, but they did not look at them, but did tell US to bring them back after the 14 day period along with our affadavits of firearms training, registration forms for all fire arms previously registered to be joint registered, State I.D.'s, your MARRIAGE LICENSE and all firearms to be joint registered and they will type out our JOINT REGISTREE'S NAME on our EXISTING FIREARMS REGISTRATIONS and stamp in the upperish right hand corner with a little stamp that says JOINT REGISTERED .

As for #3, I would have) purchased new gun myself and then registered it jointly.
Time it so you can do all old and new guns at the same time.