Not sure but the wide fee range is interesting. If you have time, you can dig in suspicious low fee rate transactions and check their transaction histories to find how much they probably spent to the Paid service of ViaBTC Transaction Accelerator.
There is no way to do that. There's a big probability that they have paid the acceleration fee from a different address.
For accelerating a transaction using their paid service, ViaBTC gives you an address to make the payment and you can send the fee from any of your addresses.