To solve this problem, you need Morgan Tsvangirai to make him president.
I'd forgot about him, isn't he in prison at the moment?
This is a bit difficult because the party is of Shona ethnicity and these are the majority in Zimbabwe
Good point however the men under Mugabe who enabled his corrupt rule and committed many crimes including the massacring of their opens in the 1980s (They had training from North Korean agents) need to be held to account in this scenario and there'd be no peace while they remain free and unpunished.
All you need is to weaken ZANU-PF and give a democratic alternative to the Shona.
The weakening part should be easy however setting up a viable democratic alternative might be a lot harder as Mugabe and ZANU-PF went out of their way in the 1980s to destroy any opposition root-and-branch.
South Africa does not want a refugee crisis and damage to their interests
Yeah and currently don't they already have OTL several million Zimbabwean refugees causing a lot of problems in South Africa?