"As for" does mean concerning, but it is not synonymous with it. It has much more limited uses. Look at what Collins says about as for:
You use as for and as to at the beginning of a sentence in order to introduce a slightly different subject that is still connected to the previous one.
1) You can use "as for" in the middle of a sentence, but not in the middle of your sentence, because you don't mean to introduce a different subject.
2) "There was a disagreement concerning the causes of the accident" is correct. You could also say, more simply, "There was a disagreement about the causes of the accident."
3) This looks like a correct use of as for, but as I said, as for means something different than you think. So your sentence starting with "as for" has a different meaning. Here's how we might use a sentence like that:
"The accident happened at noon on Friday. It shut down production for two hours. We are working to establish greater fault tolerance, so that another accident like this does not impair our work. As for the causes of the accident, there was a disagreement."
In that passage, the final sentence does not mean people disagreed about what caused the accident, it means a disagreement caused the accident. We would probably expect another sentence to follow: "As for the causes of the accident, there was a disagreement; two workers became angry and one pushed the other into the cheese extruder."
The point is, "as for" functions mostly as a transition for one subject to another. First we're talking about the consequences of the accident, then we talk about the cause.