1) Seigan is a nice thing to use. Its low Kenki cost makes it efficient to use sub-30 Kenki, it just means you need to get good with Third Eye usage.
2) Kaiten does seem to buff Higanbana's dot.
3) The best way I found to balance Kaiten Midare and Shinten is to carefully monitor your Kenki. You always want to Kaiten Midare, but you should also pepper in Shintens if you can. Best way to gauge if you can do both is making sure you always have at least 20 Kenki in the pool, for Kaiten. When you hit 45 Kenki, pop a Shinten unless Gurren is coming off of cooldown.
4) This is a source of argument for the community, I've seen. Many people will tell you to only use it when you have 3 Sens, others say whenever it's off of cooldown. What I believe? Use it when it would be most useful for you personally.