Where do we draw the line?
In practice. The only lines that really count for Aikido are found in practice. The line is very definite there.
Two words are used for practice -- renshū 練習 and keiko 稽古(a whole thread exists on this one). Keiko 稽古 connotes replaying knwon engagements (literally -- "old quarrels"), which allows for study of principles in concrete action. Kihon dosa, and kata as examples. The concept of the physical line is fundamental there.
Renshū 練習 "polish/refine learning" plays a part also. Kokyu dosa as example. That allows work on a different aspect of principles -- where the lines are intentional, attentional or conceptual. There are valid lines that exist in those areas, too, and which it is just as dangerous to cross ill-prepared as the physical line. All the better to be drawing those those lines with the same spirit and as closely as in physical practice.
Not all renshū happens in the dojo, some of it can happen here. Some of it can even be written down.