I think Jodi had two diaries, she always has things in twos or threes. Especially when she asked the police if they got the rest of her diaries...still trying to direct the show and fishing for info at the same time.
It takes a lot to unlearn how NOT to control other human beings. Imagine someone like Jodi from Day 1 with Travis, when she had to work on him for months just to "officially" date her, if he even did that. More like friends with benefits. If other people seen something was off with Jodi immediately, then Travis knew it. ....he knew, no plans to marry that. Imo, Jodi put the words 'something off about that boy' in her diary, because she had over-heard (eavesdropping) from his friends too many times about herself.
For someone like Jodi, the frustration of not being able to control Travis....she probably "thought" about killing him for a long, long time prior to planning with the gas cans, dying the hair, alibis. She's most likely a sociopath remember.
Are we sure Travis went off Jodi about a left behind picture? That doesn't seem right, I thought it was her hacking, going on about Mimi and him doing something seen by this unknown person...Jodi's lies. Steve Carroll? friend cough of Jodi's veiled blackmail threat...'40 Year Old Virgin'..."I'll out the 30 year old virgin, Travis." by exposing this little sex phone tape he didn't know she made.