Most people prefer to rush Q evolve with an early Serrated Dirk + the AD components of Kraken Slayer.

Glad to hear. :)

Kai'Sa has a very low utility score in general because she has no CC in her kit, it's not a number you should be aiming to pump up.

I noticed a while ago that it has a decent winrate and seems to be better than Stormrazor at least, which makes it worth taking into consideration, but I don't think it's better than Manamune and I don't bother talking about off-meta items unless I'm sure they're at least situationally optimal, so I likely won't be including it in the guide unless it ever takes off and starts being meta.

I've never gotten round to trying it for myself either so all I can say is that based on stats it seems okay. Sometimes 1 pro being the only one using something means that what they're doing is really stupid but I don't think this is one of those situations, I think Essence Reaver is super underrated as an option even if overall it's probably not worth using over Manamune. I'll probably try it soon though and I'll try and get back to you with my thoughts if I have anything else to say at that point. I should also mention that a lot of ADCs, me included, love rushing Berserker's Greaves which sucks with Manamune because of the delay in stacking it. If you're going to be rushing Berserker's Greaves, though whether you should be doing that is debatable, it's likely you'd want to use something other than Manamune. I think both Infinity Edge and Essence Reaver are now better than Stormrazor, so that's at least one circumstance in which you could justify using it over Manamune.

And yes sorry you're right. I've been using Sorcery secondary out of personal preference on pretty much every ADC since the new Nimbus Cloak and I hadn't noticed the meta has switched to that too. I've now updated the guide to reflect that.