The real issue here is if charges have been filled.
While unpleasant, an arrest carries very little legal weight. It simply means that the police thought that you could be guilty, but they were not sure and you were not allowed a chance to defend yourself. This means that innocent people get arrested every day, no big deal!
Now, if you have shown to the police/DA (and convinced them) that you already had sent the laptop, it shows that you did not intend to keep it itself, and it is unlikely that the DA (the one who could press charges) will continue.
If he wants to continue, you get representation by a lawyer that helps you through the trial.
To address your questions:
Is such an arrest lawful?
Probably yes, at least from the part of the police. Even if from your POV is overkill, that does not mean that they did something illegal or that you can sue them.
What options does A have?
If we are talking about defending against charges of theft, there is one: hire a lawyer.
If we are talking at getting a compensation to your troubles, next to none. Certainly none from the police. The only thing that I think could be helpful would be if your employer deceived the police about your intentions.
E.g., if you told your employer "I am busy right now but if you come to my place I will hand you the laptop, otherwise I will mail it on Monday" and your employer reported to the police that what you say was "I am keeping my laptop and you will never see it again!", then probably you could sue your employer. But then you would need evidence of what was said.
But if your employer told the police "he says he will return it but I do not believe him", then the employer was not lying to the police and he is quite safe.