1: In theory "yes", in practice "almost". You gain small amounts of exp for crafting, on the order of 100-600 usually. Refining iron ore into iron ingots, smithing ingots into arrow heads, and crafting arrowheads + wood into handmade arrows will net you around 1,200 exp or whatever total. This is monumentally small, to the point that this exp hardly counts. You get free AP as you age (each week) so you don't have to fight, in theory, you could subsist entirely on age-up AP and crafting exp, but this would be a terrible way to play. For one it'd be boring, you'd get tired of it real fast, and for second you'd miss out on a ton of easy exp sources like shadow missions. Commerce makes exp easier, as you can get somewhere around 150k in a single run once you've invested time in it (that is, enough exp to bring you from level 1 to level 25). But, you can play as mainly crafting. I've invested a lot of time and AP into refining and smithing, but I also find time to invest into my bow skills and it's paid off in spades. I've got some of the best single-hit damage in our group, as a solid magnum shot is pretty devastating, and we've built our dungeon boss battles around my ranged skills.
So, if you choose, you can pursue crafting skills primarily, but you'll have plenty of chances to work on combat skills and enjoy the dwarfy dungeon side.
2: You can rebirth to age 10 to 17. 10 has additional luck and other stats, 17 has more str, dex, and int, with the major issue being that luck cannot be gained through any real means - there's no skills to raise luck. BUT! Playing free, you can rebirth at age 20, so the vast majority of players rebirth to 17 and then rebirth for free at 20, which is a 3 week period. When you rebirth, you save all your skills, but your level goes to 0. So you can level up again and make an easy 30 AP after rebirth, allowing your skills to grow much faster. If you go for 10, then you've got two and a half months to wait, or buy (real money) a character card to rebirth when you choose.
You CAN go young, but you end up hobbling your progress. Plus it looks weird when you're 2 foot tall.
3: Yes. We're on steam groups "Dwarf Fortress: Mabinogi Chapter". We're on daily, working to progress each other and fulfill various skill and quest requirements. The others have helped me with my refining, which involves a LOT of ore to smelt, and we've been working on getting etgfrog the Ice Spear skill, though that's proving impossible at our level. We go on group dungeon runs very frequently, and will go harvesting for resources when one of us needs crafting supplies, as well as hunting down luxury items and pooling resources. Certain daunting tasks, like "collect the pages for Fireball skill book" have been a lot easier since we'll all go through and try to find the pages.
Yeah, we're very active, a solid 4 of us usually, some 7-ish who come and go overall, and in proper dwarven fashion we'll gladly and blindly follow the commands of an unseen overseer who's will is unknowable and dangerous, but we're assured that it's for the collective good and we're all about communist assistance.