Plagiarizing is copying without credit. You're not copying -- you came up with the same ideas independently.
Go for it.
Go for it.
It's a pretty untapped space where even the simplest ideas are a big deal at this point, so I don't want to be charged with ripping him off.
Should I delay my release until there's a substantial improvement on his?
Some people will make that claim whatever you do. I wouldn't worry about it too much if I were you.