You launched a formal complaint complete with witnesses and documentation which was investigated for 10 weeks while you were paid for doing nothing, and then your complaints were found to be groundless, even your witnesses did not back you up.

Is this co-worker relationship salvageable?

No. You put multiple people into a difficult position. Things will never return to how they were. More constructive to worry about being forced out altogether. You should have spent the 10 weeks job searching, suggest you get onto that asap.

Then everything changed. I don't know why, but I think Steve regretted
giving me the position.

I'm guessing there's more to this than just a sudden change of attitude. It's unfortunate you didn't talk to Steve to find out why.

How can I salvage/repair this situation and co-worker relationship?

Before anyone suggests it, yes my line-manager has a manager of his
own, unfortunately they are very good friends outside of work, so I've
tried that route with no success.

You could try talking with him and seeing if you can determine why he changed. Perhaps you have something to apologize for here. If so, it would be a good time to do that now. But your relationship may already be beyond the point of repair.

It appears that instead of talking to him directly, you chose to create a 6 month log and then took it to HR as a formal complaint. And apparently, you also went over his head and tried to complain to his boss.

It's not clear why you chose that route first rather than trying to repair the relationship, but what's done is done.

You might need to watch your back now, and start thinking about finding a new job or at least a transfer to work with another manager.

The other answers suggest you've made a mistake at some point. I tend to disagree: the situation was lost when Steve thought he could teach you to become a senior programmer and then couldn't keep his temper as he found out he couldn't do it.

Being a good programmer

...doesn't automatically mean you have good teaching skills.

I'm a programmer myself. I teach in a hackerspace in my spare time and on a few days a year in a school. Teaching requires time and patience. Also you need a whole different skill-set. Just being able to program won't cut it. If your student is to make big steps, you need to prepare your lessons.

Steve tried to recruit because he needs more people on his team to help do his job. So when he started to teach you instead, it meant he would have even less time to do his 'real' job.

He must have found out that teaching someone else isn't as easy as he hoped and, as his frustration grew, he began to lose his temper.

Don't take it too hard on yourself

It probably isn't even anyone's fault. Take a deep breath. Look around for a new job. Take care it is the kind job you always wanted to do. Take care that your future colleagues are the kind of people you want to work with.

OP I'm sorry but I think that the "investigation" was basically over in a week. You were given 9 weeks to begin searching for a new job, which you did not take the hint for. I say this because software developers are not a resource that is wasted lightly.

Update your resume and start looking immediately, additionally note how your other coworkers are treating you; you'll need to get references from someone at this company who is not Steve or in the HR hierarchy who have dismissed your claims of harassment.

Unfortunately you may need to find a new job now, but at least your job title has "senior" in it and you have plenty of time to search. Remember that you only need to put "Senior Software Developer at Company X 20XX-present" on your CV.

You mention that you are logging your boss' behaviour, so keep doing that. In particular your requests for meetings, so that if they try to claim your performance is poor later you can cite the lack of work and repeated requests for it as the reason. At least in Europe they can't fire you when the boss is doing that to you, it would be constructive dismissal.

As to salvaging your current situation, could you ask for an internal transfer? If not, your only real option is to go back to HR and complain about a hostile work environment where you are unable to get any work or progress your career, due to retaliation. That is a serious situation, you could escalate it to a tribunal, so they should deal with it in a way that doesn't disadvantage you.

Check the work legislation of your country. In Spain, an employer must provide adequate tasks to their employees. If your employer is not providing you any task, this could be consider as mobbing -which is what they are actually doing, to force you to leave the company. An employee can claim to have work.
Of course, you are in a bad position. If I were in your situation and this was a small company, I would be thinking about leaving. But if this is a big company, maybe my dream company, then being moved to another department would be ok for me. If you speak with HR and expose this as a case of mobbing, then they may take you more seriously. No company wants to be in the media because of mobbing. On the other hand, if you are thinking about quitting, this can give you a better position to negotiate an exit with a compensation.

Your manager acted petty (or maybe you did not understand the situation completely but let's assume you are right), but your question is unjustified now. If you wanted to salvage the relation, that should have been your first approach rather than going to HR. By going to HR you made an implicit judgment that the relationship was not salvageable in the first place.

However, even though you could (and should) have picked a better response to your manager's behavior, I don't think it is all only your fault.

It is unfortunate that your colleagues couched when asked formally by HR. Also, it is unfortunate that your manager's manager preferred taking his friend's side rather than taking the right side.

Since the relation is not salvageable unless someone has a sudden change of heart and this guy in question is your manager, the best direction is to get a new job.

It may be bitter tasting but it is the next best thing that can happen to you.

It depends if salvageable means get back to the way it was before you filed the complaint, improve things so they're better than when you filed the complaint, or just salvageable as in you can stick around and not get fired, I'd say they are all possible. Getting to do what it takes to salvage will determine how possible it really is, though.

If you can find out what Steve's core needs are and find a way to tell him what is core needs are, you will disarm him. This is a core tenet of nonviolent communication. For example, if he has a need for competence and effectiveness more than anything, you could find a way to express you see this as a need of his and it will be very relieving for him to hear. It will create some connection between the two of you. That may be enough to salvage things.

If you are on your way out, as a lot of other posts advise, I suggest at least trying this. You don't have much to lose and it will be good practice for the next time you are in a conflict. Building the case against him, while done out of the need for peace and equality, in his perspective is the opposite of seeing where he is coming from. If you can see where he is coming from, you have the best chance of salvaging the situation. He may feel threatened by you and not trust you after that happened. If he was merely frustrated he couldn't teach you, now he is probably scared of you. If you succeeded in getting him to change by forcing him to through HR, he still would not like you. You may even be asking the same question you are now, if things can be salvaged after filing complaints against coworkers.

If you are going to use tactics of pursuing justice from authorities who have powers you don't, it won't truly get him to change. In the Art of War it talks about taking the enemy whole. Had you won with HR, Steve still would have greatly disliked you. This is the equivalent to defeating an enemy and taking their land, but burning all their fields of wheat in the process. Better to take the enemy and their fields of wheat than take the enemy and burn their fields of wheat.

But for it being salvageable, I'll go with yes, it can be. Just see his needs as a person and empathize/express you see them. That connection is rare in the business world. It may be hard to have the chance at doing that now, though. If you can't get close to him and have the opportunity to do this, maybe it's not worth staying there. I live in a small market, so I can empathize with wanting to salvage things as opposed to just moving on.