Edit: Wall of text incoming!
Anyway - thoughts, suggestions, advice on what i did wrong etc. welcome. Suggestions to use danger rooms will be ignored.
So i buckled down and tried actually training dwarfs in weapon usage instead of putting ten on inactive with most labours disabled and a barracks. Drawing from forum wisdom, i started with a two-dwarf squad, complete newbies (born in the fort and drafted within a year after growing up). I micromanaged them a bit by leaving their jobs on and giving them a month of free time after every three months of training, to improve their mood - eating in the legendary dining room, talking with family and friends, being satisfied at work. Expectedly, they took a long time to level up - five and a half years until they turned elite.
Then i split them up and gave each a fresh recruit to train. Those were elite after two and a half years. Not exactly speedy, but if you could train ten dwarfs simultaneously, that should amount to a pretty good buildup, esp. considering training gives a fairly well-rounded skill distribution with good defensive skills and very very good attributes.
After the newly-trained dwarfs were elite, i put them into their own two-dwarf squad, where they quickly sparred up to legendary and beyond. One of the established teachers got a fresh recruit to train, the other got two. And now things got interesting: i moved the single recruit over to the sparring squad after three years and a season, when he was still only at 'accomplished', i.e. not yet elite. It took more than half a year in the squad to finally advance to elite. The recruit pair is by now, after almost four years, at 'talented' and 'proficient' in their chosen weapon. In addition, the sparring squad went from 2/3 sparring to perhaps 10% sparring. The main culprit seems to be the spread of minor 'unarmed combat' skills - the more skilled dwarfs eventually pick up biting, kicking and misc. object user skill, and will teach those skills. To exacerbate the issue, with this wide spread and the massive time investments in demonstrations instead of sparring practice, old skills inevitably start to rust, which prompts the loonies to schedule even more godforsaken demonstrations to unrust them.
Another problem is that with increasing number of dwarfs in a squad, interruptions of training routines become ever more frequent; dwarfs will leave the training zone to eat, drink or sleep, and each such abscondication is almost guaranteed to interrupt the ongoing practice, prompting a whole new 'organise training/demonstration'. These interruptions are slightly mitigated when the squad is allowed to carry food/drink (although carrying food will only reduce the food-related breaks, which are the least frequent and disruptive anyway, by one half and will block happiness from the dining room when off-duty), but they are still irritating with two dwarfs and quite annoying with three.
Compared to marksdwarf training via archery ranges, this direct melee training is a whole lot quicker - in the five years which produced two legendaries, and one each master, talented, proficient and adequate melee dwarf, the five-dwarf archery squad produced one great, two accomplished and an adept and talented archer (the last two were still on hauling duty). An off-duty melee squad of six dwarfs was plugging away the whole twelve years, and they went from talented-to-expert to one Grand Master, one Great *** and the rest Accomplished, so expectedly, actual training is orders of magnitude faster, not to mention that the off-duty squad got only weapon and fighter skill anywhere notable, esp. their defences were practically nonexistent.
I guess that fighting living targets should develop weapon skills much quicker, so backing that up with a solid training regimen should allow somewhat faster training of a strong military. I wonder if large squad with two or more teachers might work - two teachers and one student proved completely useless, the teachers kept starting/aborting competing demonstrations over and over, and the student didn't learn a thing.
P.S.: mood of training dwarfs is a bit of a problem. I did all this in a completely sealed-off fort (twenty dwarf diplomats on the surface currently) with no invaders, and the non-elite soldiers were generally 'content', with great or grand bedrooms and year-round training. The only additional bad thought that ever came up was the very mild booze monotony, which caused no harm, but in an actually contested fort, a training dwarf could easily be sent over the edge by losing a friend or family. Giving them regular time off (every three or four months) to enjoy the dining room, do some workshop jobs etc. can keep them happy or better fairly consistently.