I serious doubt if N9 and those two lower end-lumias are playing in the same league. Just hardware-wise I think you would get more from your N9 than the mentioned lumias. My sister is also using an N9, and she lives happily without a terminal.

If you are careful in installing apps, it would easily survive > 1 day. How do you define heavy usage? The only downside for a heavy user is that the battery is not removable. Common IM services are available, you may want to check extraplugins project.

Considering N9 is not supported in your country, you probably are on your own if your device is f**ked up. However the probability of getting a faulty unit is probably comparable to hitting a jackpot (: Just make sure you are getting a new unit then it should be fine.

Just out of curiousity, if you are considering WP8, why not Android then?!