Having spoken to the California EV rebate program people directly on this issue, there are sample documents on their web site. Tesla has its own category for sample documents, apparently recognizing that you don't just go to the lot and take home a car. The purchase order date may be the controlling date. My DS was sure the confirmation date was controlling. I even asked if I could take care of the paperwork now, to be sure the date on it would meet the requirements even if my car wasn't quite ready yet. I was told the paperwork would happen at delivery.
My conclusion is that, at this point, so late in March, there is really nothing anyone can do to speed up their delivery to make the cutoff on the 28th and the information available is murky, but promising. My thought would be to take delivery by the 28th, if possible, and if not, talk to the California EV rebate people to see what documentation could point to a pre-expiration date.
The rebate application does not need to be filled out immediately, so I don't think the registration date is quite as important an issue. There is also the requirement that you own the car for 30 months. Clearly, that is not going to happen before March 28, 2016.