[RPG] Can Druids use Sylvan to speak with animals without using the spell

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.

Yes, but the game doesn't tell you which. As long as your Druid isn't a newborn baby, they've likely seen some beasts in their life (especially during and after their Druid training!), and therefore can Wild Shape into those beasts.

Your background is probably the best guide to what creatures you've likely seen, as your background will imply a region that's familiar to you. Any beast common to the region in which you grew up and trained is a beast you've likely seen many times, and uncommon ones are likely to have been seen once or twice too.

This is all between you and your DM, of course, because the game doesn't rigidly define a list or rigidly define a way to create one. You're two reasonable people who enjoy playing games together though, so I'm sure you'll figure out an enjoyable, reasonable set of beasts that you can agree makes sense for a nature-loving outdoorsy person to have ever seen in their life.