Sir, is it ONLY for assessees who have to do efiling mandatorily? or for all assessees to whom 31.7.12 duedate is applicable?

No. By reading the notification, It implies that it is made for the All assessee as whole in addition to assessees for whom the Efiling is mandetory.

Notification says : " in respect of assessees who are liable to file such returns "  and in the first sentence it says " in consideration of the fact that the e-filing of returns for a specified category of individuals and HUF has been made mandatory"  . So can it be interpreted as SUCH refers to mandatory efiling Returns?

https://www.incometaxindia.gov.in/home.asp

No, the Word AND is used before the sentence "further in consideration of the fact that the e-filing of returns for a specified category of individuals and HUF has been made mandatory,

Further, On the ICAI website, it is notified that the, the due date for the assessees who are liable for filing return by 31/07/2012 is extended upto 31/08/2012 & this covers all assessee.

No. This is applicable to specified category of individuals and HUF only.

Now on 01-08-2012, Central Board of Direct Taxes has clarified that the extended 'due date' of filing of returns to 31st August, 2012 is for those returns which were to be e-filed by 31st July, 2012.