What determines whether a person can be in charge of a certain development position?

Obviously Writers, Designers, and Sound Engineers can be the lead in their respective fields, but sometimes I can have a Sound Engineer take the lead on graphics. Sometimes I can have Hacker A perform any job while Hacker B can only perform two. What determines who you assign to each development position?

What their current job is isn't the criteria. It's whether they have any job ranking in writer/designer/sound engineer that matters.

Correct, as long as they are at least level 1 in the related job, they will be visible.

And you definitely should not keep Newb Ownerton! He has a terribly small energy bar :)