for the first 2 question I don t really clear.
For the third I can suggest you something.
For any xdata it would get more cpu time to access these group of data via
pointer register.Then the first to decision is how often you use for the defined variable to get less time of cpu.
Another ,when power down mode or standby the internal ram will not be lost in contrast xdata will have got an unexpected data from these so usually it need to reconfigure from nv memory.
if you need any variable under these mode you d better implement on internal ram.


best regards