The problem with planetary landing is the explosion of data. If you want to render something the same way for every visitor, that means that even if it uses PG for the initial planet, afterwards it needs to contain all the seed information for that procedural generation when the next visitor comes.
Not quite sure whether I'm not understanding you, or whether you don't quite get how PG works. The data won't explode once someone has been there - in fact there is no data to start with - just algorithms fed by seeds and some constants. The storage requirements are trivial, considering the number of places to render. Everything is done on the fly, every time.
Allowing you to actually interact with the environment would make the amount of data even larger, since now you would also need to track what each commander has done on which planet and add that to the engine whenever the next commander visits that planet.
Yes, it would add data in the form of overwrites to the algorithms, but depending on how it is implemented, and the range of things it is possible to do, the overhead would not have to be huge - a few kb per player per location probably.
Adding cities, let alone unique inhabitants would be nearly impossible. So my guess is it will probably be pretty landscapes (without player interaction) with a couple of starports, kind of like we already have a couple of different types of space stations.
Not at all, but we probably wouldn't get a full-sized city to explore in complete detail. I imagine it would comprise a starport area with shops etc that we can use, and maybe some hand-coded areas on some worlds, but we will be confined there for quarantine and immigration reasons. We'll see the rest of the cityscape from a distance and be able to fly over it at a minimum height, but not actually go there.
I have a feeling FD will amaze us with the planetscapes just as much as they have with the galaxy.
There's also no reason that inhabitants can't be generated, either, other than rendering limitations. Theoretically they could PG the entire NPC human population of the galaxy - why not? They've done 400 billion systems, probably containing a trillion objects - why not a trillion NPCs?