1. You never need 2. You always need one. OK, in the end you have two or more for the whole flight. But you just have to have one at a time. How many there are in total for one flight is irrelevant. The change point, where both have the same distance, is the PNR, as you said.
2. Also for that you can have as many as you want. You just need a valid flight plan, and this one has to work according to the Etops rules. You can change your flight plan of course, and some do, you can take a whole bunch of fligth plans with you if you should need it. As soon as you leave inhabited area for a certain distance, you get into Etops area, you have to be covered with a valid alternate (90, 120 or what ever flight times away).
3. Destination alternates have to be within 60 minutes, at least all legal frameworks I flew with, and surely EASA/JAR. Etops is for enroute planning, not for destination alternates. If your destination alternate is further away than 60 minutes, you have none, and you have to calculate with isolated destination.
hth,
Dani