Osek and Mekda ... enslaved all their life :(

I agree - compared to the impression we got in the Original Quest the three are surprisingly grown up. I have two possible explanations:

1. The "it's not done by Wendy" effect - and the Tales of Love and Loss have more serious inconsistencies as the age of the characters.

2. These elves are obviously immortals. I remember reading about Zahran (in the BotC novel "Colors") that pureblooded elves grew up slower, at least mentally and emotionally, and were considered to be younglings much longer as their wolfblooded agemates. When I remember right it was said Zarhan was about 100 years old - but the "same age" as much later born Rahnee considering maturity.
For me it makes sense - so I'm going with it.

Following ideas:
1. The influence of the Palace - in case a high reproduction rate was common for this tribe.
2. They had lost a valid number of tribemates in an accident or such and Recognition made surte to refill the numbers. The Wolfriders had a similar baby boom around the Madcoil incident.
3. Rockshaping was (almost) as common in this tribe as gliding was for the Gliders.
4. When my thought about the "extended youth" of pureblooded elves is right there can be decades between the single births.

It's assumed the the survivors of the Ice Elves met the Two-Spear's splinter tribe at some point. The tribes merged and became the ancestors of the Go-Backs.

I don't have an good idea why Mekda was mutilated worse. It can be each of the reasons you suggested. Okay, that's my 2 cents:

As I answered in the other thread, Mekda was kept back in the north to serve a group of Guttlecraw's Trolls who stayed back to hold the fort when the rest of the tribe wandered south.
1. She had to take all the bad temper, cruelty born of boredom and sadism of these Trolls alone.
2. She was alone - she might have been desperate enough to rebel more, like you suppose, and was punished more often.
3. Or - she was simply not good or fast enough. The Trolls seemed to handle subordination and breakdown the same way.

Who says they have not tried?

But when there is any truth in the saying "Recognition knows!" it would know better than creating another enslaved elf.

Mekda was kept back in the north to serve a group of Guttlecraw's Trolls who stayed back to hold the fort when the rest of the tribe wandered south.

Easy for me:
1. More rockshapers = faster (and easier) work = more tunnels = more ore = more gold and gemstone.
2. Those Troll did not USE the rockshapers - they USED them UP. Greymung ordered replacements!