Basically, the way I see it, Rondo started the season with a new motion, pushing the ball from above his forehead with his forearm and wrist (a la Mark Price). But lately - or not so lately, it's been going for awhile - he's been returning to his old way of shooting initiating the motion with the ball to the side of his head and extending the arm. It seems to me he's been experimenting with the 2 motions. If I were seeing him always following the same motion, then I'd be less concerned (or I wouldn't be concerned at all), but I doubt he'll ever improve significantly without being consistent with his mechanics.
This is all 100% true. But I see it more that he's fighting his way through getting the old way out of his system. He definitely was using a different motion, as you described, with his elbow in and not extended, but has also definitely regressed. Though I see the regression more in his mid range shot than his free throw shooting. I see him before free throws doing a correction motion before the shot, purposely trying to teach himself to keep the arm in and the ball higher. It seems to keep his free throw form a bit more consistent but the correction at times looks like it is throwing him off balance a bit.
I can only go by what I read from a couple of sources about the time he worked with Price. I never saw Price's comment on the situation but I'll take your word for it. Still, switching to a whole new motion is something that takes time and I think we have to be patient here. If at the end of the year the monthly splits aren't showing some results on progression, then definitely bring in someone to work with him further. But work with what you have and what has been changed because starting over again, IMO, would be detrimental. Adjust what he has.
I like what he has been taught and I think it can work for him but he, more than anyone else, is going to have to be the one that is more disciplined about what he's doing. I think he's gotten the proper coaching and changes. He needs to implement them properly. Unfortunately, he was doing things so poorly for so long overcoming what is natural to him is going to be a tough, long journey.