Hi..
Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern.
As per your complain and presentation of symptoms tenderness of ride side of neck and back of mouth along with a history of Bruxism that is grinding of teeth at night can be due to grinding habit which can cause continuous 
fatigue to the muscles and it can also lead to muscle 
stiffness and inflammation leading to tenderness..
Another cause that commonly causes tenderness of neck, throat and back of mouth can be due to bacterial or viral 
throat infection..
Nothing to worry, consult an oral physician and get evaluated and in case if the tenderness is due to muscles then 
muscle relaxant like Chlorzoxasone and painkiller like 
Ibuprofen can be advised along with warm compresses.
If it is due to infection antibiotics, painkillers and warm saline gargles will relieve..
Hope this information helps..
Thanks and regards..
Dr.Honey Nandwani Arora.