If a new client is requested to be added to an LHP session that was carried out long before, could that accomplish healing for them in the present? Will this bring about a form of retrocausal healing that differs from carrying out an LHP session in the present that requests retrocausal healing of past causal influences to occur?

The difference will only be in the details of how the healing request is launched. Either approach can be effective in getting healing started in the past that will reach forward to the present and bear fruit.