1. The owner can file a suit for eviction even now from abroad itself by giving a power of attorney document to a close relative to conduct the case in his absence.
You should not be carried away by the threats posed by the company, let them do whatever they want to including the complaint, that can be dealt with separately, but make sure that the company is paying you all dues and vacating the property since they turned out to be bad tenants hence they are liable to be evicted.
2. They cannot approach court for violations in construction, if at all they have any grievances in this regard, they may have to approach concerned local civic body,m but they cannot do that now because they had been under utilisation of the said property till now or continue to use the same as a tenant, and now their complaint in this regard may not be entertained.
Hence you dont be scared by such threats, you can ask them to proceed but be firm in your decision to vacate them.
3. Don't stretch your imaginations for committing such blunder mistakes, they would be of no use, it may not be advisable to resort to such unenforceable ideas.
4. Why do you worry about his defence, instead you concentrate in your own prosecution based on the grounds you rely upon and be undeterred while prosecuting the case against him.
5. The parent of the owner may withdraw his statement for NOC mentioning that he was forced to sign such a NOC under pressure and coercion, further the authorization should be by a GPA deed and not on a plain paper to make it valid.