So,
1. It is normal for you to be accused of infringement. Just reply normally.
The patent you bought is invalid. If you can get it back through administrative litigation, it is best. If there is no chance, it depends on whether you can get the patent transfer fee back.
In view of your ability to handle patent affairs reflected in your question, it is recommended to entrust professionals to handle it on your behalf.
Zhong Kui talks about patent blog.