The signature of 12th marksheets are irrelevant for verification process. These marksheet are only meant for DOB, Name and as a qualification proof.
The signature that gets verified is as per your application form that you submitted for the exam/vacancy. Further a valid photo id proof is required for identification purposes only, signatures are not verified.
The same is applicable for any exams in India.
Even in case your verification process fails due to some or other reason, you can still take the test/exam by submitting in written, that you will be liable for any fraudulent activity, and might have to provide finger print on the same written document. In this way, you could still take the exam, and your verification could be done later.
Basically you must look like the photo you placed on the application form, reach the venue and know your roll number to take the exam. All other aspect could be managed later. However, please do not feel encouraged to commit fraud as it will lead to serious legal actions,