Joy,
Second-Corporal or as it became Lance-Corporal, was only a temporary rank back then, worth a couple more pennies a day for more hassle. It wasn't unusual to bounce around the ranks then either.
The Medal Roll for 1877-8-9, was available a book, edited by D.R. Forsyth. However there is a cd-rom which is currently available of it.
The material at Kew used to be on microfilm, but it has been some time since I had to refer to it.
The Pay & Muster rolls of the 88th are at Kew. These are quarterly returns John McCusker's name would indeed appear in them, as would any stoppages to his pay for whatever reason.
By searching them between 1878 - 1881 you be able to establish the date, and I use the expression that would be recorded in the Pay & Muster Roll, he became Non-Effective that also normally gives details of his Next-of-Kin to whom any personal items and outstanding pay/prize money would have been sent. So they are well worth a look from that aspect.
The Battle of Quintana as mentioned in a number of the links is now referred to as Centane, try a search on that version and you should find some modern photographs of the area as it is now. I've got some engravings of the 1877-8 campaign from the illustrated newspapers of the time, should they be of interest please PM me.
Regards,
Isandlwana