If this is a matter of which county had precedence for warrants, then wherever the murder took place is the county that he would have to be held at. If VCSO arrested him for a lesser crime and found that he had a warrant for murder, then they had no choice but to hand him over to Lake Mary. Murder takes precedence over lesser crimes. I'm guessing, but I assume they did not make a formal arrest until after he was taken back to Lake Mary, it was just a matter of transferring custody from one county to the other. If this is the way it went down, that lawyer is full of baloney.
However, if he made a statement to VCSO regarding the murder, and they gave that statement to Lake Mary, then I'm not sure whether that will be admissible or not. I'm not a lawyer but I do know that transferring custody from one county to another is done all the time, and perfectly legal. My son did a lot of transports of prisoners back years ago, and sometimes to other states. He told me once that it is more or less an informal arrest when they meet to transfer custody of the prisoner. There are papers to be signed by both counties/jurisdictions.
I would have to think that these two LE agencies were well aware of the boundaries and did not knowingly do anything illegal.