Well, what you propose is probably a viable strategy, but it’s very specialized and the fact that it threatens both the UK and USSR doesn’t mean it’s the best strategy and we should all adopt it. Also, my question would be whether the dual threat and the immediate capture of the Leningrad objective can offset all the income from the south you’re foregoing and all the money you spent on transports without taking London.
Back to the original point: unless you build full naval for a turn as in the Sealion fake that you propose, UK is going to realize pretty fast that it isn’t the target whether or not Germany has built a Romania IC, so the “giveaway” of Germany’s strategy isn’t really an argument against the Romania major.
Keep in mind that buying the factory on turn 2 after an ambiguous buy on turn 1 does slow down the UK for a turn by forcing them to account for the possibility of Sealion. Also, if the UK lets down their guard, you could consider a late Sealion under some circumstances. In this case, the Romania IC could help because you can drop 10 infantry a turn to link up with your forces as you retreat from Moscow or to defend your territory if you decide to send the initial wave of troops south to the Middle East. The factory also opens up more strategies that don’t involve spending every dime on the Eastern front. For example, you could strengthen your air force instead of buying expensive ground units.
What I’m getting at is that the Romania major may actually help you target the UK in the long run, or at least keep the option open.