No shortcut down, although I think there’s an easier route down than you’re taking I think. You need to find the elevator back up to where you fought Artorias. If you approach Manus from that direction, there are fewer humanity sprites and fewer of the crazy fellas
And to summon Sif, you need to save him first, have you done that?
So for scrappy, while you’re in the chasm, keep an eye out for a ghostly version of Alvina, that cat from Darkwood before you fight Did. She leads the way. If you’re coming from the bonfire, stick to high ground and I think you’ll see her eventually. There’s a spot where the ground is illusory and falls away I think. Anyways, if you like around up top you’ll find him eventually
As for the key, you need to back into the Township. There’s a big room with loads of the crazy townsfolk (I think a couple sorcerers and the rest are the normal ones), and the keys on the first floor of that room I think
When you get to the bit with the long fallen pillar that leads down to the boss go up and past it killing to town guy throwing dark magic at you. You need to head down past that (so keep the dark chasm on you right) and down there is where you will find the lift back up to the area outside of Artorius as well as the illusory wall that leads to Sif
You should have everything now to just make a quick boss run each time. When you come out the main lift just head right and follow the colourful stone lights to get down.
I figured if I got Art down Manus would go too but he is killing me pretty easily. I could do it with a lot of patience I can see but should level then maybe. I can’t seem to get my weapon any higher, it never gives me the opportunity. That said I have picked up lots of weapon type shard thing near Manus.