I've been thinking it's stupid using Matt as an information source because just because he's been her best friend since they were kids doesn't mean he knows everything. Jeremy could easily bust Katherine and I think it should be him like Tyler says something and Jeremy gets suspicious so he starts doing little things that Kat wouldn't pick up on and bringing up things that Matt would know about but not nearly as well as Jeremy would.
For example they could have gone on a camping trip when they were younger and came back early saying Elena got sick and then Jeremy could say "Yeah that's what we told Matt and everyone but the truth is Elena went into the forest to pee and got poison ivy where no one wants poison ivy she swore she would kill me if I told anyone the truth."
Jeremy could even screw with Katherine's head and interogate her and thow her off with questions like...
I mean there are so many things Jeremy as Elena's younger brother would know that Matt wouldn't things only a brother would know things so embarassing Elena would never write them down in her diary assuming they happened when she had a diary. Which is why he's the biggest threat to Katherine and I can't believe the writers aren't exploiting that.