1. Not many servers so population is fine. Red side will have fewer players. There might be too many people in the newbie areas for blue side. *cough*
2. Scrapper, brutes, MMs, and sentinels.
3. There's A LOT of story content. There's only so many factions and they are broken via level ranges, but the game ran for like 9 years, so there's a ton of content.
4. Just go to a place like Croatoa at some point. Hint, it is the Halloween zone they made official.
5. MMs can be busier to handle although since the pets to the attacking, it might be boring. Peacebringers and Warshades were originally unlocked after hitting 50. Every mission there is someone there designed to kill you asap.
6. IO related salvage and recipes are worth keeping. Little else until end game content happens.
7. I'm trying to recall. Via missions, absolutely.
8. I never found it that bad. I've done a fair bit of soloing while taking heroes to 50. If you team all the time, you can get there at a reasonable pace.
9. Some servers are specified for EU, RP, PVP. I go where the people go so Torchbearer.