the biggest manipulation via self-playing with split votes is 50% curation reward instead of 40%, which is same as linear reward.
I do not understand this.
My conclusion was there is no effective split-vote abuse mechanics but the system incentivises unwanted behaviour (looking for spots where 5:01 vote gives more rewards than 4:59 - this cannot happen without a priori split of rewards between the two pools).
What keeps a large stakeholder (today) from authoring a random comment via a separate account then posting an early vote from yet a different account, then posting a large last minute upvote.
Lack of incentive - they are better off dropping the big upvote after 5 minutes.
Unless you assume they are afraid of being downvoted. That is why the new system is expected to have a window before payouts (like 1 day) when upvotes are no longer possible and only downvotes can happen.