@warwick Sorry if I’m misunderstanding your question here! So, I think what your example shows is not possible with the current rules.
In your example, are forces A, B, and C all starting at the same sea zone? Or saying that force A is just trying to invade Sumatra, while force B screens in the sea zone adjacent to India, while force C invades India, right? Are there enemy naval units also in the Sumatra sea zone? I believe so if I’m understanding correctly.
If no enemy naval units there, this isn’t even a screening move that requires the rule to come into play. That’s just a standard divide your forces to attack wherever you want rule, and then splitting an amphibiously assault force between what kills an existing navy and what would then amphibiously assault India.
If there are enemy naval units in the Sumatra sea zone, I would say that above example isn’t possible. The rule 9.7(b) states that: “The screening force engages in combat with any Enemy units present during combat phase. If the screening force wins (i.e. it eliminates all Enemy units or they retreat), the moving force immediately moves into the next sea zone where it may conduct a second regular naval combat or an amphibious invasion (but not both) To move into another sea zone each ship must have remaining movement points and expends them all in doing so”.
This move would require a screen of the naval units in the Sumatra sea zone, making the next move to the adjacent India sea zone one where you can conduct another naval combat or an amphibiously assault, but not both. What would happen here is Force B screens the naval force in the Sumatra sea zone. Assuming Force B is victorious, Force A amphibiously assaults Sumatra, and Force C then moves to the adjacent India sea zone and can conduct a naval combat there if one is possible, and if no naval combat is possible, may amphibiously assault the mainland (but couldn’t do both).
Regardless, I can certainly see how clarification could be needed!