If not will they be dropped from the pool in 21 days?
Where did you get the 21 days from?
By default, transactions are dropped from the mempool, if they stay unconfirmed for 14 days.
(Nodes that have dropped unconfirmed transaction from their mempool can receive them again, if they are rebroadcasted.)
Sounds frustrating if you'll wait this long.
I just wanted to know if those paid services of Bitcoin transaction accelerators will help in this case like ViaBTC since it has many partners in a mining pool.
..if it does not confirm in the next 9 days, will simply contact the sender after.
He is a reputable member of the other domainers forum and I hope he will not make the same mistake in the future.
Next time you should wait at least 3 confirmations before making such deal, it's a safe indication that you are making a good transactions but anyway since it was a reputable buyer, it might worth it to wait.