You have probably already seen
https://www.findagrave.com/memorial/191679993/robert-lusk-carson and noted that death year on the memorial stone is 1896, whereas the closest matching death in timescale , which does seem likely to be for the same person, on civil registration is for a Robert Carson, with no middle name noted, at Moyaver, who died on same date, 5th February, but in 1897. Will at PRONI for this Robert of the 1897 death, also includes no middle name.
When Eliza Jane Carson married Henry Smyth in 1868, her father's name was also recorded as Robert Carson, with no middle name mentioned.
Also, Margaret Simpson nee Carson, looks to have actually died on 20th September1894, aged 80, rather than the day, year and age on the memorial.
Do you have any actual record that notes your chap with the Lusk middle name?