They have a valid contract. They could have moved up to HK years ago but they decided to accept the terms and conditions offered which required them to remain in ADL for a few years of instruction. The quid pro quo was expat terms.
cxhk - they have seniority - and if you read COS you will see that CX can second you. There are precedents to 'employed but seconded' without it being Check and Training.
There is no breach of contract for them to be currently employed and seconded. The breach of contract will be if CX doesn't honour its side of the contract. So far that hasn't happened.