You probably should of just started a new thread, as this one is stickied for returning / new players. But anyway,
2: Yes (depends on level and gear of course, but many have solo+merc Naggy which has a 52 level limit)
3: No matter what server, getting groups at lower level is difficult. The new 51/50 server (Mayong) starts at 51 level, and just recently opened, so more likely than not, much easier to get a group there, even starting out.
4: Depends on individual players. I do much of the old-world content. DoN missions have seemed to fallen to the way-side other than the Progression tasks, but I know several that still do LDoNs either for points, fun, tribute items, or adventure stone progression.
5: Yes, there are benefits. DoN gear is somewhat obsolete, but if you're just starting out and low on funds, it's fairly easy to skill up in smithing and/or tailoring and be able to farm and make your own equipment that is descent, even though not the best in-game. As it has always been with tradeskills, it depends on the particular time frame. Currently Jewel Crafting is still a good skill, but again new Defiant jewelry has lessoned it's usefulness. The newly implemented non-caster Research tradeskill is also useful, but much more difficult and expensive to skill up in that the caster Research, but the non-caster hybrids and melee can make Discipline Tomes as well as their particuar class spells, that the caster Researchers can not. Poison Making has been revamped and non-Rogue usuable poisons are fairly sought after at the moment. Alchemy has always been a useful tradeskill. Each has it's points, and popularity varying over time.