Yes latent venom is 100s duration at all skill levels.
It works on bosses.
Yes, Velnia monkey cards work with all wugu poisons, since patch on May 23. At 'bug fix section of patchnotes: KR patchnote link
QS3 wugu1 rogue2 hacka2 is ok single target and very mobile with dash and fast multi hit attack, pair it with bonus damage buffs (priest & chaplain buffs, enchanter buff). This build will be lacking for high AoE. You could consider taking ranger1 for the critical shot buff (+50% crit damage buff without downtime) to combo with rogue sneak hit and high mobility. Rogue1+ranger1 is possible but sacrifices rogue2 backstab and sneakhit downtime will be 10s.
If you are looking for highest sustained damage, I am not sure if this build will outdamage a fletcher3 based build (A2 r1 wugushi1 fletcher3 hacka2? or qs3 fletcher3 hacka2).