And there is another big problem with the doppelganger theory in that if you play as a warlock you will see your patron, if you are a paladin you can break you oath...those things could not happen to a doppelganger because a doppelganger would only mimic a warlock, it would not actually have a pact with a patron. A doppelganger would only mimic a Paladin, it would not have any oath that it would need to uphold or that it could break.
That is true. There is a way to solve this issue by explaining that the magic used by warlocks mc is not normal but it psychic magic that is directly powered by the mindflayers themselves. Mindflayers are extremely powerful, and can create nearly limitless supply of psychic magic-this explanation is also full of issues but not necessarily unworkable.
Regarding Paladin, a Paladin's oath is born of their genuine belief in their oath, since this doppler is so deeply convinced they are a real person, it is within the realm of possibility that although normally it would not work. The doppler is more tav then doppler. It's why all the origins mcs have something weird with how their powers work.
Shadow: Shar clerics can not heal or use restoration spells from my memory. Yet she can
Astarion: a vampire not harmed by sun
Kar: claims she is from hell and fierce warrior,but can not take on a lvl 4 paladin. (Real Kal could, she is a dop and is weak)
Gale: Needs to feed on magic, as you said, arcane magic is hard to simulate. So mindflayers trick gale.
Laz: not recognized as a warrior by creche, despite being known (likely detect she is an imposter)
Wyll: has memory of glory days, but is utterly weak, super easy impersonation
Dark urge: a doppler who failed to identify / transform. So filled with a desire to dominate all like a minflayer.
