I'm too tired to parse this, but here's my (completely fucking failed) attempts to get Valley to pure white. If you've done any tendency experiments, feel free to share.
Here's the (outdated) info from the wiki:
http://demonssouls.wikidot.com/world-tendency
Some test results post-patch - it seems that it is possible that going to the Nexus causes a shift towards Neutral for all worlds. We need more testing to determine how this is working.
Started a fresh loaded game, all worlds were Neutral.
Died 5x in Valley 5-1 using Ephemeral Eyes to resurrect, but went back to Nexus a few times to check tendency - possibly caused the increased number of deaths to get it to full black? Region went to full Black.
Went into 5-1 and STAYED in the Valley clearing out Black Phantoms in 5-1 and 5-2, as well as the Primeval Demon. Returned to Nexus. Valley was Neutral, not White.
Attempted to coop in 5-3, dropping coop stone in front of 5-3 and 5-4 bosses. Killed them a combined total of 6 times, suiciding after each revive in Storms (went black after 3), then Tower (black after 3), then Stonefang (one death, partial black). After each coop kill, returned to Nexus to check tendency - Valley stayed Neutral. ALSO: Character was reviving in Storms/Tower due to dying there as a body, possible this affected the world shift in Valley?
Frustrated with the lack of progress in Valley, with Stonefang still partial black and Tower/Storms full black, left Valley, went to Tower 3-1 to kill BP Rydell. Arrived and found... Blue Rydell. Checked World Tendency map - Stonefang was neutral, Tower was one off full black, Storms was still full black. Possible that leaving Valley caused Neutral shift?
Quit the game WHILE in Tower, reloaded, checked tendency - all worlds showed the same as before. Died in Tower to a BP Squid, left for Nexus - all worlds showed Neutral. Appears that the world status is saved IF you're IN a world after a quit, but it resets to Neutral from the quit as soon as you leave that world.