I can't imagine for what reason, but can a Torah Scroll be opened while still in the Aron Kodesh? In other words, must a Torah Scroll always be removed from the Aron Kodesh in order to open the Torah Scroll?
There is no documented reason not to open a Torah Scroll while still inside the Aron haKodesh.
Actually, many Sefardim communities seem to have the custom of opening the Torah Scroll while still inside the Aaron haKodesh before taking it out and parading it to the Bimah; often still open so all can peer into it.
Personally I roll the Sefer Torah to its correct place while it's standing in the Aaron HaKodesh and nobody has ever commented on the [im]propriety of this. Admittedly, לא ראיתי אינו ראיה - missing evidence is not a [good] proof.