Saw this on hbd as well...
I've only used kaffir once, and I crushed (crunched and torn with my hands) two large leaves and put in at the end of the boil. Low, but noticeable aroma. I would suppose you could add it in any of the ways you mentioned.
I haven't dealt with lemongrass in brewing. When used for cooking, I like to cook it for about 5-8 min right before serving the dish (usually some type of curry). I would suppose doing the same for brewing would work well.
Good luck, and I look forward to the other responses as well.