app.executeCommand: Next Keyframe & Previous Keyframe

I would not use menu commands in scripting unless it is really necessary. It is much more reliable to use

nearestKeyIndex and keyTime etc. to find the times of keys and set the comp's time to that value. The menu commands rely on many assumptions, like a comp being active, and don't give helpful error when this is not the case. And they don't work at all, if Ae is running headless.

Those two lines by themselves don't do anything. You need of course to fetch the selected layers/ properties and check the state to contextualize the operation, which is what the script you mention does. Why try to reinvent the wheel?

Mylenium