Paizo has studiously avoided publishing a single official word on psionics. The only psionics rules are from a third party, Dreamscarred Press, but are in the online Pathfinder SRD. According to those rules, if you are using magic-psionics transparency, then you treat powers just like spells or spell-like abilities, so yes, detect magic would detect and identify them. The question reduces to "How does that work with a spell-like ability?" Transparency means "the same" so yes. Spellcraft would let you ID a power as it was being used; Detect Magic would identify an active power's school (note those are two different things). If you use magic-psi transparency then psi is magic and all the same rules apply.
They include the separate PsiCraft skill because you don't have to use "magic-psionics transparency". That's just the default. A GM could, if they wanted, make them opaque to each other, at which point a Psicraft skill is necessary. Also because some classes might have Psicraft as a class skill that don't have Spellcraft.
You're asking for which is more powerful - is that really what you care about, or do you just want an interesting character? The number one rule of multiclassing is "never lose spellcasting levels." You've already pretty depowered the character as a Rog3/WW1 even with Magical Knack. I imagine none of the heavy CharOp folks have answered this question yet because your initial build has already provoked them into running about their residences screaming like enraged howler monkeys (rogue, 1 strike; losing spellcasting levels, 2 strikes). If you're looking for superpower, the train has left the station. But if you just want an interesting character to play, read on (though you really should specify what it is you want your character to be able to do/be like...)
In isolation there's a legitimate sorcerer vs witch debate, but if you're a third level rogue who has taken one level in witch, taking anything other than more witch is a severe power compromise - the "third strike." You're getting +2 CL in one class from Magical Knack and then if you were to switch, effectively taking -1 spellcasting level - pretty much losing as much as you're getting. So in this case, "definitely Witch."
As you move on, instead of Sorcerer I'd stick with White-haired Witch (seems like it synergizes well with rogue anyway), or go into Arcane Trickster after a couple levels in Witch - it'll keep full casting progression and is designed to highly synergize with rogue. But never lose a spellcasting level again. Look at it this way, if you were to switch to Sorcerer and be like "woot I want to throw spells", at level 10 you're barely going to be throwing fireballs when normal level 10 spellcasters are really melting faces.
There was a character in my last Pathfinder game who was a Rogue 2/Shadow Oracle 9, that worked out OK (he had a limited times a day super backstab ability and had oracle-boosted stealth stats) so it synergized with rogue well, plus invisibility and major image). So you can multiclass, and even use rogue, but definitely stay away from even more multiclassing. Pathfinder made specific design choices to back away from 3.5's "combination of 6 classes for optimization syndrome" and usually staying single-class is as strong if not stronger than a combo, and the more combo you put in generally the greater a disadvantage you'll have over your comrades.
Best Answer
No, it can't. Animate dead and similar spells typically have a duration of instantaneous. This means the magic is only present at their creation (and for more powerful spells/spellcasters, an aura may linger for a time). After that, the negative energy has already infused the creature and made it undead - this is a "natural" (stretching the term quite a bit) state for an undead creature, and requires no ongoing enchantment to keep it functioning.
A lot of confusion on this issue stems from the fact that the only way mortals (read: your PCs and the vast majority of NPCs) can manipulate positive and negative energy is through magic. But a dead body infused with negative energy in other ways can become undead without any "magic". A deity capable of manipulating negative energy directly can just push the energy into the creature. A place where lots of dark things have happened can naturally attract negative energy, and people who die there may rise as undead. Some undead can "infect" living targets with negative energy carried by a natural vector, like a disease or poison (like plague zombies). None of these require any magic to create the undead.
Basically, animate dead just "starts the engine", the creatures can "run" without any more magic.