I made a squad of seven axedwarfs and used the "Danger Room" technique to get them all up to Legendary in Fighting, Dodge, Shield, Armor and Axe. Even though this can probably be considered an exploit (but seriously, the military system is pretty broken, you'll get sieged before you can even get your first proper squad up if you have the "train" normally They just stand around preparing for demonstration 80% of the time, 10% they actually train and the remaining 10% they are on break, anyways..) but I can tell you that these 7 dwarves in iron gear have successful obliterated two goblin sieges (probably around 10 gobbos each in total) without any outside help and not taking one single hit. They have done the same with a forgotten beast, slayed that bad boy in two seconds. Didn't even have time to zoom in on the fight. None of them even has a scratch.
So yeah the problem here is that either you do it the intended way and it wont work very well or you do it exploitive way and it becomes waaaay to effective. I'd wish there was some middle ground here.
I fully armor and arm my dwarves, and they do quite fine. To have them spar, they need a squad leader who knows weapons to teach them. Before ambushes and sieges, my fully armed and armored green troops sit on their ass and do worthless demonstrations. When the ambushes and sieges hit and the soldiers who survive gain weapon skills, I see sparring happen all the time. If you want some more or less experienced troops at least by the first few ambushes or siege, then send them into the cavern (armored!) or outdoors to kill wildlife with training weapons. I usually do the former.
As for crossbowmen not training, keep in mind that the exp gained from training is inferior to the amount gained by shooting at live targets. Either sent them behind fortifications against siege forces to train them up (2 years of 3 sieges got my marksdwarves who had no skill all the way up to Adept, Expert, and even two Accomplished) or capture troops with cage traps (I understand how people don't like spamming cage traps. I don't either, so I limit myself to building only 5, indoors, and each of them at least three tiles apart) and let them be pin cushioned. This will only have a problem if you rely entirely on marksdwarves with no traps.
Anyway, sorry I kept posting at this thread. But I do feel the need to explain that the military is really working quite fine.