Mackinlay was found by a Channel 4 investigation to have declared a considerable amount of election expenditure as belonging to his party's national campaign when it should allegedly have been declared as local spending. Adding this expenditure to his expenses return would have put him a long way over the legal spending limit.
During the election campaign Nigel Farage and his team warned that Mackinlay was spending more money than he was allowed and we will soon find out if a judge agrees. Many other Conservative MPs were found in the Channel 4 investigation to have done the same thing but the CPS only brought a case again Mackinlay which suggests either a show trial given the importance of that election or that Mackinlay and his team were doing more than just following advice from the party.
Even if he is found to have cheated the 2015 election a judge would have no power to remove Mackinlay from his post as there has been another election unless he is sentenced to more than 3 months in prison.