I considered skill requirements and may still edit them in later, but for now the only skill requirement is on the feat. And I have also edited my second post to include the requisite feat.
Don't be sad Grod, when I can think of an appropriate capstone I will put one in. I just haven't heard a good suggestion or had that "Eureka!" moment yet.
The soul singer and allies only gain the benefits of essentia invested while the essentia is still there. You can kind of think of a Songmeld as a soulmeld with a duration. So if you start a performance with 2 points invested and then later move 1 to a soulmeld, the song immediately becomes weaker, so Lingering Song wouldn't have any special effect.
I may still take up Shneeky's suggestion of having the Songmeld correspond to body slots, but I still need to think about it a bit more.
It's possible. At the very least I'll consider it if I can get enough people interested in the class.
Not necessarily. You can enter the class as a Bard/Totemist.