Is Antpool's accelerator free though? The only free transaction accelerator I'm aware of that actually is working is the one from ViaBTC and that one requires a minimum transaction fee.
No and it also looked broken yesterday. It wouldn't work for me when I wanted to see what the fee would he to push something through.
It's also worth noting that once you sign a zero fee transaction in electrum. No one else will broadcast it unless you do a nonstandard transaction on Bitcoin core which isn't something to do lightly.
You can double spend a no fee transaction also. You guys are forgetting that a node won't broadcast a transaction that is a double spend but if it's a zero fee transaction most nodes don't accept it so it can be double spent in the mempool as long as you send it to a node that accepts it. Only one transaction will ultimately be added to a block though.