If Yahoo was in china, yes no investor would in their right mind think that the share would hold value of the company's share in another. But here in the US, golden parachutes can go to the shareholders, so...
Bottom line is I don't think there is a way for Ma to screw Yahoo shareholders out of their stake in his company.
If the planned IPO is for the 120B as mentioned in a few places than Yahoo value is almost 0 without it , let say Between 3-5B . If they will pay 30% more than today price they actually will get yahoo for around 12-15 B .
The problem is that after the IPO Yahoo will trade for 45 ( assuming the figures of IPO ) .
Believe Ma will play a game. He will not want others to have the kind of control YHOO would have given the present layout. Suspect YHOO mgmt. thinks 1-2 steps ahead while Ma thinks 4-5 steps ahead. Price of YHOO will be engineered down to Ma's advantage. But then... I have been wrong before. Good luck.