Querry related to deduction under section 80 u

As per S. 80U deduction can be availed for 50k (100k in case of severe disability,which means 80% or more of the disability. Note that disability has been defined and it includes blindness,which also has been defined. You said its 100% blindness so exemption should be of 100k for your uncle.[Do check whats mentioned in certificate]

2]You have to obtain the certificate from MEDICAL AUTHORITY,this authority can be either state or central. The authority MUST BE NOTIFIED for the purpose of "Persons With Disabilities (Equal Opportunities,Protections of Right and full Participation) Act, 1995
JUST CONFIRM THAT THEY ARE NOTIFIED,OK.
I am presuming that in your case it has been certified by State govt. of Maharashtra

For certain diseases certificate is given for fixed period and after expiry of that period re-assessment is required & If the condition of disability requires reassessment then fresh certificate is to be obtained.[Check the certificates validity]

3.)You need to file the copy of certificate, as for attesting it, am sorry am not sure of it but it should not be necessary as its by govt. authority and ITR will be signed (Digitally or manually),attested one should also be accepted
Still i'll advice you confirm your AO.