You can definitely buy multiple apts in Shanghai as a foreigner currently.
I have the green books to prove it
Must be different districts though.
1) Yes
2) Can definitely lease, although foreigners are supposed to live in their primary residence
You will need to pay tax if you lease it, unless the renter doesn't need a fapiao (rare).
Agency will usually do this for you, and take off the rental.
3) As others have said, not strictly enforced.
As I have found, not a problem to own lots of property in China, or Shanghai.
See my note about different districts above.
Even in the same district its possible, although a little bit mafan.
Tip - Chinese ID's don't change, but passport numbers do. Chinese system doesn't cater for that fact.
Not saying any more.
4) Probable, but irrelevant.
5) < 5 years need to pay capital gains tax.
*Unless its your first apt, and its your primary residence*, in which case you don't need to pay capital gains tax. Essentially you don't need to pay it...
6) Given that the chinese do the same, probably no real age limit.
Not sure offhand though!
Note that different districts are tougher / easier about these things. Also different people have different experiences.
Paul and I both recently did apt stuff in Putuo Qu - for me, they were very easy/flexible, for him, everything had to be 100% jin chang.
So YMMV!