Diff is at 2791 now... indicating somewhere between your 10% and 15%.... try 13%, I think I read that somewhere as the "official" number.
Yup, the only thing broken in diff adjustment is its design. But otherwise it "works" as intended.
so it sounds like difficulty is increasing as planned. The problem was initial design having initial difficulty of 1. And that doesn't seem like that much of a problem. It does heavily favor whoever was mining for these initial low difficulty rates. As throwing huge amounts of processing power at the chain would not increase difficulty heavily.
If the initial difficulty was 10, the difficulty right now would be 10x what it is now. If it started at 100 it would be 100x what is now. This was probably an unplanned problem due to the variables stated above.
The only problem I see with having the hard 10-15% difficulty increase cap is the chain does not adjust well to large amounts of processing power being added. That coupled with the uneven % increases in the negative direction, and the "attack" mentioned a few posts back of strobing/throttling a large botnet onto the chain seem like it could become a problem. Not so much a huge problem, just that a large amount of processing power could be used on the chain for a longer amount of time due to the retargetting scheme - which would mean potential attackers could "harvest" more out of the chain without affecting the difficulty as much.
I also don't think the huge # of stales at the start was an attack. It more sounds like the extremely low difficulty of 1 coupled with a huge influx of unexpected miners caused a lot more problems than expected. If you are generating a block every 1s - it's going to be real hard to keep the whole network on the same chain. Propogation time alone of the blocks would probably take longer than generating a new block - and that was more the main issue it seems at the start of this chain.
As for central authority and % skimming of generated blocks that's a whole other issue not related to security, and more related to trust and design. As people have been posting, that really is an issue with trust and with who controls that wallet.
Claims of 51% attack preventions - I don't believe there is enough evidence or proof here to refute or back up those claims. But I'd be wary of assuming this is true without proof. But I also wouldn't go out of my way to say it's not there.... There is no proof for either side to be true, just claims that their side is true.... Show me a 51% network control double spend and I'll believe it doesn't have protection, or show us/explain to us the code of how a 51% attack couldn't succeed. But without proof on either side my bias would be to assume it isn't true because there's no gain, only potential loss by assuming it is true without proof.
edit: difficulty started at 8? or you could think of it as difficulty starting at 6.03 after first retarget?
6 * (1.13^69) = 27578.0688