Imo try selling the mule. Like make a random level 3, and when you go to sell a chunk of gold just toss it on a mule and sell the mule. Just looks like you tossed it and it got hacked.
The money probably didn't appear because it was f2p and hadn't waited the 24 hour trade hold. If you get the account a bond and make it a member it should be able to see the money when it appears.
On a side note, drop trading can be sketchy in the case of dcing/isp problems. Would honestly just trade it off your main unless you're moving bills daily.
That sounds about right. Old starter mains are probably the best accounts to use as mules. But I don't know for sure. Maybe you should PM some of the big gold traders and see what they do.
Having old accounts is the best option as muling goes, train like a very old account and chances of you getting banned are slim unless you are careless with trades to the sellers.