can automated ban system detect troll builds?

Nope

No and it shouldn't really. Some people buy those out of frustration while Nexus is falling while they are dead. I have seen players do that even in LCS. There would be way too many false positives.

I don't know. I don't think anyone not a rioter would know either. It would make sense that they could detect such. >2 tears or >1 boots and >3+ minutes left in game should trigger some punishment, but again i don't know.

#YES

The automated system can detect some of the most common troll builds, and actually won't punish if they're purchased in the last minute or so of the game as a frustration buy.

It's not perfect, and sure it misses quite a few cases, but those cases are borderline that an automated system cannot reliably detect.

The extreme ones? Probably. The more subtle ones like AD Soraka? Probably not.

The problem with detecting troll builds is patches. Think about it, what if they tomorrow remove the "unique" quality on tears. It's probably not a great build, but it could work on some champions. And that is the extreme kind of troll build. So you would sort of have to make the system "forget" all learned patterns on items whenever anything is changed about those items... which is inherently bad for a system that entirely relies on remembering patterns.