check again the definition of your 2 MSC’s : external LAC, external CI, routing towards other MSC, etc. There is still a problem in there.
in cell reselection, the MSC is not involved at all : its an autonomous choice done by the MS. It proves that your external cells are correctly defined in your OMCR
during HO, the MSC has to carry info to the other MSC. I think this is not currently done in your network. Which proves that the MSC’s are misconfigured.