(a) What happens if someone who holds a lifetime UK PPL (A) and decides to apply for a EASA PPL (A) after 8th April 2018?
So long as you meet the requirements of Annex II to Regulation 1178/2011 you will succeed.
(c) Can a UK PPL gain grandfather rights on EASA aircraft?
A grandfather right is the ability to continue exercising a privilege once held following a change in regulation that might require additional qualifications to gain that privilege. As there were no EASA aircraft prior to 2012 there are no privileges to be grandfathered.