I'm not a CDer, but my SO presents very well and also reverts back to guy mode easily. So if you don't mind I'll put in my
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Keeping up the diet and skin care routine will not give you away in guy mode. Cleansing, exfoliating, and moisturizing your skin will give you a clearer and smoother complexion which is enviable in anyone. As for the diet, the changes you might make in a few weeks won't make that big of a difference in either mode although it is always a good idea to eat sensibly.
I'm not sure about the work outs though, unless you are seriously pumping iron daily. But I have a son who was working out seriously and it took a while to build up his muscle to the point he could bench press 300 lbs (he played offensive tackle on his HS football team), and it took months and months for his upper arm bulk to decrease once he stopped. I don't think a few weeks either way will make that big of a difference.
The nails may give you away depending on how long you like them, but could you not use glue-on nails in femme mode, or choose a nail length that could go either way? Medium length and well manicured nails are not unheard of in men and if they are painted in femme mode, they look equally nice. I've seen dazzling women wear shortish, squared off brightly painted red nails with quite an effect. It looks clean and contemporary.
You didn't mention the hair or the eye brows. Arm hair can be clipped and not completely shaven (even GGs have arm hair) and this will not look unusual in guy mode. If you like to shave your back and chest, it will not show through under a shirt. In the summer time I don't know if you wear shorts in guy mode, but if you do, lots of guys into sports have smooth shaven legs. And eyebrows can be trimmed in a way that would suit in either guy or femme mode.
The trick, as silkandsatin said, is to learn to be comfortable grooming yourself in guy mode and not look at it as strictly a girly thing to do.