Imagine someone said to you "Go take $100 out of that wallet that is lying there, use it to buy bitcoin, and send the bitcoin to my address. It's totally fine, that's definitely my wallet and my money, I just can't do it myself." Would you do it?

This is essentially what has happened. Someone has given you log in details to a third party's (or parties') account(s). You have logged in to the account of these innocent people, stolen their money, used it to buy bitcoin, and sent it to the scammer. If you are prosecuted for this, then your lawyer is right. The excuse of "Someone else told me to do it" isn't exactly going to stand up in court, even if you have proof of them telling you to do it.

I've also received an offer like yours before. It happened via Telegram and thankfully I didn't bit his/her bait since I'm not negotiating to any person online who pm me first out of nowhere; without seeing his/her identity. The only way to track that wallet down is to know if it's an exchange's address so you may contact that exchange for them to help and assists you. However, your chances of recovering it is too small.

If the coins were transferred to a mixing service then tracking them will be no longer possible. Why were you accussed of phishing? Can't you provide correnspondence with the person who asked you to buy the coins to the police?