OnePlus 3 repair issue
Your repair guy is either ignorant /or taking you for a ride or both

Bad battery story by the repair guy is flimsy. 


You had good battery life - 8 hours is actually pretty decent
Batteries are not balloons - if they are swollen, they stay swollen and don't regain original shape (on no load as you mention). See picture of  badly swollen battery


Swollen battery spin test is one way of making sure it is bad. For details, see this nice answer with embedded video

3) Batteries don't make round holes if defective. They puncture, or explode at worst

You are being taken for a ride, sorry for that, but there is no polite way of saying that. 

The only sensible option I see is take your phone to an authorized or reputed repair agency and forget this guy and the money lost