Here lately I've been tinkering with an invisible computer, trying to add some features to it. Right now, I want to enable skilling. (Sims would be able to gain any skill using the object) In theory, once I get a working BHAV I should be able to clone it 6 times and change the literal value for each BHAV.

I tried importing the "Interaction- Play" BHAV and adding a few command lines. But ingame, the sims can't react to it at all. It's probably broken, but I don't know where. Is this the best way to do it, or would it be easier to write a simpler BHAV from scratch?

At a first glance, I noticed that you imported the BHAV "Interaction - Play - TEST" (and not the "Interaction - Play", which is the main action BHAV). However, I see that you have edited that BHAV to act like an action BHAV (you'd better delete that "TEST" from the name, then).
But the interaction can be started by sims only if called from within the TTAB; and in your TTAB, besides the different descriptions, the "Play" interaction still points to the semiglobal "Interaction - Play" (0x2004), not to you imported BHAV (0x1006).

Okay. In the TTAB, I connected a new pie string [it says something like: Long Range Sensor Sweep (Logic)] to the new interaction BHAV (active) and to the "Interaction - Play - Test" I imported (guardian). It still acts broken, the sims can't interact with it.

If the option doesn't appear in the pie menu, double check:
- if in the TTAB the options for the ages (adult, teen, elder ets) are set up correctly; the option "debug" must NOT be checked;
- if the guardian BHAV, for whatever reason, doesn't act as it is supposed to (in order to know if this is the case, put 0x0000 in the guardian BHAV field and check if the option appear in the pie menu: if so, the guardian BHAV is the culprit).