Agreed, this is the key point - I suspect Solberg goes straight to the first team, and Katsuta drives alongside Sami in the 'b-team', apart from on rallies where Ogier skips.
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I think they'll have to be a little careful over eligibility, so their 4th driver can remain eligible to drive for manu points in the main team if there's an injury/illness in the top three. I think if you drive for manu points in the second team, that stops you doing it in the first team for the rest of the season.
I assume Evans and Ogier (when entered) will always be nominated for manu points, but it'll be interesting to see who else. I expect the de facto "#3" driver will be whoever else is nominated for manu points at Monte. And whoever they additionally nominate in Sweden (assuming Ogier skips it) will be #4.