1) The IT return for LTCG will have to be filed by your wife, as this is her income and not your income.
2) As your wife is not a business person/professional, she needs to file ITR 2.
3) Make an application to the Sub-registrar requesting for providing valuation as on 1/1/81. If you do not still get or the sub-registrar is unable to provide valuation, you may make a best estimate of the valuation as on 1/1/81.
4) As the property value sold is only Rs 20 Lakhs, I suggest that you may deposit the entire amount in Capital Gains bonds, which have a lock in period of 3 years. This money can be withdrawn after 3 years and used for any purpose, as you may wish. Even if there is an error in valuation or difference of opinion by the assessing offer, it has no material impact, as the entire capital gains will any way be exempt.
5) You can save LTCG if you invest the entire sale proceeds in the property u/s 54F, provided the house is registered in her name. However, it will create problems for you in getting housing loan and claim deductions towards interest and repayment of principal amounts in your IT Returns.
In view of the above, I suggest that you should invest in LTCG Bonds and withdraw the amount after the lock in period of 3 years.