@nen-suer
1) You should give him the Sword, he will use it to attack you, defeat him and you'll gain back the sword. Be careful though, this will shift the WT tendency to Black, but it's kind of unavoidable, since he'll attack you anyway, even if you don't give him the sword.
If you don't give him the sword and kill him after he attacks, your CT will also shift to Black, as this is counted as an attack to a named NPC.
2) You did well in killing Yurt, if left alone, he would kill one random named NPC in Nexus for every boss that you defeat, so you could end up with dead on Nexus if you didn't kill Yurt
3) http://demonssouls.wikidot.com/upgrade Here's a list with each of the stones and where to get them
About Pure Darkmoonstones in particular, they are dropped by the two Crystal Geckos in 4-3, near where the Stormruler sword is. One of them is in plain sight, while the other is hidden by a patch of grass, so be careful.
Edit - Yeah, to farm souls and Stone's of Ephemeral Eyes while avoiding to go back to the nexus, I was suiciding off the cliff near the start of 4-2. When I revived, I sniped the first manta that passed by, but when I was looking for my bloodstain, which the game stupidly hid behind one of the pillars to the side of the stairway, I was snipped by a manta that didn't made that particular attack sound >_<
I'm just a trophy whore for Demon's Souls, this game deserves it :P
Current PC Build
CPU - i7 8700K 3.7 GHz (4.7 GHz turbo) 6 cores OC'd to 5.2 GHz with Watercooling (Hydro Series H110i) | MB - Gigabyte Z370 HD3P ATX | Gigabyte GTX 1080ti Gaming OC BLACK 11G (1657 MHz Boost Core / 11010 MHz Memory) | RAM - Corsair DIMM 32GB DDR4, 2400 MHz | PSU - Corsair CX650M (80+ Bronze) 650W | Audio - Asus Essence STX II 7.1 | Monitor - Samsung U28E590D 4K UHD, Freesync, 1 ms, 60 Hz, 28"
