Is it worth going into human form to fight dark phantom Mildred in the swamps of BLIGHT TOWN?

You get the Meat Cleaver weapon from her, some humanity and a bunch of souls (like 9K) plus the ability to summon her for the upcoming boss fight of Blightown. I believe it's worth it (not for the summoning) for the drops.

203 new posts later I finally got the game.

Well, I beat the hydra in the basin, now I'm off to go back to blight town to check out the pyromancer I missed and do the option boss there (I was curious what was down that hole by the bell, I assumed it was the same place I fought Quelagg (or whatever) and, considering I had just killed her and had 20k souls, I didn't want to risk dying by fall or due to a new enemy.

So, I'm trying to get back down there via valley of drakes -> blight town but I can't seem to make my way to the lower level (I'm at the water wheel). Is the only way to that lower area to go to blight town via the depths?

You can take the water wheel down in Blightown back to the lower levels (the swamp). You'll be taken to a place near the bonfire of the swamps are of Blightown and from there is just a straight run forward to Quelaag's place.

Woohoo getting this game tomorrow. I am super hyped. Never played Demon's Souls before and don't really know what to expect but i watched some vids and it looks very interesting.
What would a good starter class be? I don't really want to use magic.

Classes don't matter too much at all, but if you really don't want to ever use magic, don't start as pyromancer, sorcerer or....cleric(?) as they have some spells as default. Beyond that I don't really have any recommendations as the starting classes all sorta stop mattering about 2-5 hours into the game.

So they play all the same? Same control layout / mechanics for every single class?

Pretty much, I am playing as a thief and just learned Pyromancy and sorcery anyway. I'd recommend going as either a thief or maybe pyromancer, then deprived on the second playthrough(Although that's harder with no armour, hence the second playthrough, or unless you've played this type of game before). I've started as thief and got the "master key" gift automatically which means that I could pick another gift on top.

I hear pyromancer is a good starting class, and I did play with it for a bit, fire spell was pretty useful.

But I found getting used to dodging/rolling etc a bit hard seeing as I've never really played this type of game before, either way if you get a drake sword and can single handedly wield it = you're set for some time.

Yeah, the controls are pretty much the same. If you want to cast magic you use the same button as if you want to do a normal attack. The battle strategies and the playstyle vary wildly if you're a magic style, melee style or ranged style. 

As EncodedNybble said, the classes stop mattering after 2-5 hours, they're more of templates, giving you better starting stats on the stats you want to develop first. You could go with a Deprived if you want to use a make-it-yourself class, since all it's stats are balanced. The Deprived is considered a pro class though, because it starts with no equipment whatsoever, so the first hour or so is reliant mostly on dodges and perfect timing.