I wonder what shenanigans they get up to?

Probably made a club that investigates paranormal activities.

Now we need a time traveler

(baku)nyu class...

...and also need a Tsuruya-san equivalent, just because, and definitely no Asakura types.

I wonder if they know about their friend's rather...dubious taste in partners.

Tonari-chan is less than 5 years from the kid. compare that to Ai-chan and Onii-san who are suggested to be more than a decade apart in one of the manga chapters...

That seems like an argument that will hold up in court

for Ai-chan or for Tonari-chan? because Tonari-chan is a minor too just like the kid, while Ai is a minor while Onii-san is not. though i don't know if Romeo and Juliet laws exist in japan, the fact remains that she is still closer in age to the kid than Onii-san is to Ai