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For anyone who's interested, this argument can't be used at the south end of the Quarry line. Quarry Line trains are treated as if they went via Earlswood, but they never stop in the station (not least because the track layout means they can't stop there without reversing). A journey from Redhill to London via Earlswood would inevitably involve a double back through Earlswood, so isn't allowed.
I agree - it's not valid to go from Redhill to London via Earlswood on a Redhill-London Terminals season.
As you say, the absence of fast line platforms and the position of the crossover mean that you can't actually do the whole journey - you'd be forced into an invalid double-back having travelled south through Earlswood.
Nevertheless, I'm tempted to argue that the season ticket has underlying validity from Earlswood north which is potentially relevant to the OP's question.
The mapped routes from Redhill Group to London Group include the route via Earlswood-(Quarry)-London. And even it wasn't mapped, the route going via Earlswood then Quarry line is within 3m of the shortest route from Redhill to London.
As it's a season ticket, all sub-routes of the permitted route are valid (even if the whole journey isn't possible in practice.)
So I think the OP only needs a ticket from Brighton as far as Earlswood to use Quarry line. (And since the prices are the same to Redhill , a Brighton-Redhill ticket should cover the OP for both routes.)