couldn't he have single-handedly done away with the Priraka and led the Inika to the Mask of Light?
I looked into this in-depth somewhere in the revelation topic, and basically Velika's performance actually seems to "hint" at him being a GB in disguise (keeping in mind Greg likely did not decide this until more recently, though). I don't have time to find the link to my post right now, if someone else wants to that's cool; I analyzed quote after quote in detail on it.
The basic reason why he didn't singlehandedly interfere is that until the mission of the Great Spirit was complete, Velika needed to remain in hiding. Velika's 2006 portrayals very much fit this; he did the maximum a smart Matoran would be expected to, and basically drove the events in ways that minimized memory of his influence in others, making it seem like his ideas came from others like Garan. His riddle technique is basically how he did this, I showed. But when you look more deeply at the conversations, it's clear that nearly all the big ideas originated in his mind, as far as what drove 2006 plot.
So basically, he DID defeat the Piraka and helped the Inika to the Mask of Life, just in a more GB-style subtle way.
Couldn't he have dealt with the Barraki and other members of the pit that were causing trouble?
The Voyatoran were not involved in the 2007 plot, so not without blowing his cover. They were still up on Voya Nui. They did come inside for the blowing up of the stone cord, but there would have been no opportunity then to do anything about the Barraki. And basically, the Toa needed no further aid there; they knew what to do. Defeat whoever got the mask.
Or done something about Teridax?
The GBs had already programmed the destiny system to do something about him, probably without realizing it at the time, but he was supposed to help reform Spherus Magna. There was really nothing further that Velika could have done for that. However, he may have been subtly working behind the scenes in ways we haven't heard of to set the stage for Mata Nui's victory there, once he realized Makuta had gone haywire (in the sense of not following his programming as intended).
For example, some have theorized that he was responsible for the Blade Burrowers.
I'm not quite sure how powerful he was but i'm sure he could have done certain things to help rather than sit back and basically enjoy the show.
As as far as we have evidence for, he had no powers himself, but merely the knowledge of a GB. So it was vital that at all times he preserve his cover. If anyone were to realize he wasn't just some random Matoran he could find himself in major trouble fast with the less savory organizations or even individuals. (Possibly including the Order, heh.)
Also not sure if this stuff was already talked about somewhere and i'm just making a dupe topic sorry if I am.
It was all talked out in the revelation topic, but I don't mind having a topic specifically about the question of how it fits with old canon at this point, as it's been a while. That topic's primary purpose is to give general reactions to the news, so it's okay IMO now.
Actually, it was all talked out conclusively in the contest topics before Greg even announced it, in fact. That was why most of us had decided Velika was our most likely theory, as he fit the rules Greg gave for it the best.
So we're sure none of his powers could've transferred with his consciousness?
Well, we don't even know yet if GB "powers" are merely access code interfaces with their protodermic creations, like when those two GBs made the Matoran unable to see them, or when the Lifer switched off Vezon's Olmak. That could be technological, and in that case Velika could technically access them. But there may very well be limits to what they can do. For example, perhaps he could switch off enemy powers, and perhaps the "don't see me" command only works if the person to be hidden has the body of a GB.
If that was the case, then enemies could still hurt him the old-fashioned way. And he'd need to sleep sometime, so he couldn't necessarily switch off all powers of attackers; a surprise attack could get him.
So it all boils down to the same basic rule that he must take extreme care to blend in.
Also information is great power had he been able to prove he was indeed a GB (maybe difficult but probably possible) to the others he for sure would've been able to command them at least through fear
That's possible, but that would be a last resort, because he'd be blowing his cover in the process.
Also yes he's a neutral in general but they created Mata Nui to do something for them wouldn't he have wanted it to succeed then?
It does seem that he wanted Mata Nui to succeed, as he didn't start "cleaning house" until the mission was a success. However, he apparently broke the will of the other GBs in going into hiding in there without their knowledge, so we don't really know how much he is for and how much against. After all, he's going around killing people that he should see as good guys now (or about to, at the fortress), so there's definitely a few screws loose.
More importantly, we still don't know what the grand goal he is now pursuing is.