This is a right sheet show.
if the lights are marked as class 2 then they don't require an earth.
if the guy installing them thinks they don't comply with class2 requirements (as marked), they should not fit them.
mastic is not likely to provide a suitable IP rating for the enclosure, a suitable gland and seal would be required.
any outdoor light like that will end up leaking eventually and a small hole in the bottom will stop it filling up with water.
If I employ a subcontractor to do installation work for me, then the brief is clear.
Do it my way or walk away, the chance to voice opinions or change how it is done is when discussing the upcoming job, not afterwards.