I’m not an HR officer, but I was thinking about almost the same questions. I think HR is among those who should first join the transaction and the last to leave. The big issues are associated with high positions as one officer will be needed in a post-merger function/segment and one or two officers to deputize. High position officers start looking around for other entities and they leave as soon as they can if the management did not communicate them to stay specially those with high potential to achieve. The direct answer from my point of view for the selection process is “before the 2 entities are legally merged”. Wrong or delay in the communication and selection may cost a lot as the entity may lose big size portfolios with qualified/experienced/reputable officers leaving the company because of miscommunication.
External consultants for HR function is helpful but not essential. The more information and analyses HR has about the employees the less the need to HR consultants.