> 1. Suppose the OP did not take the source code files, but memorized the source code and later recalled it from memory. Would that be theft?

It is copyright infringement, yes. That's why clean room implementations are done by someone who hasn't seen the original source code,

https://en.m.wikipedia.org/wiki/Clean_room_design

For the other examples, it depends but I'm pretty sure a copyright infringement case for either of them wouldn't be immediately thrown out. IANAL but I do know that law is quite fuzzy.