In relation to the CGT issue, this is a self assessment tax, and so on a practical basis, your parents can put in their return, include any enhancement expenditure, and if the revenue accept their calculation, thats the end of it. In making the return, theoretically you do not have to send receipts. In some cases there is an audit ( supposedly randomly selected) and if there is an audit, they may be asked for receipts etc. Again, on a practical level, the revenue are usually fairly ok to deal with, and they dont always insist on receipts- for eg I have had a client who did numerous bits of enhancement to a house, and never got receipts, and in dealing with her audited CGT return, the revenue accepted an estimate from a local builder who visited the property, had the renovations pointed out to him, and then gave his estimate on how much they would have cost- this tallied with the clients account, and the revenue accepted it. I would imagine it is up to the individual tax office. So realistically they should include all enhancement expenditure ( this only includes things like fixtures and major renovation- not decoration ) and the professional fees on purchase and sale ( including auctioneers and legal fees) and stamp duty on purchase if there was any.
As to the grant being means tested, I'm afraid I dont know- could you contact the relevant department and make an enquiry?