I totally agree. It is very easy and not at all complicated for me to believe that CL was involved given his statements to LE along with Erin's statements to her husband, mother, and friend in TN.
First, Erin's statement to her husband that she was going to JTNP combined with the times she was expected to be gone (from 7 to 1600 hours) perfectly fit CL's stated alibi *and* timeline for that day (it's really amazing bad luck for him that he left at approximately 7:30 and arrived back home at 1600 hours if he wasn't involved). And, IIRC, CL stated that he was in JTNP on Saturday hunting.
Second, her text(s) (I'm assuming there were back and forth texts due to the nature of text convos) to her friend in TN about how she was going hunting with CL that Saturday fit perfectly with CL's stated activities on Saturday. (Again, it's really amazing bad luck for him if he wasn't involved that Erin told her friend she was going hunting with him on Saturday and then he happened to go hunting.)
Third, Erin's mother indicated in an article that her daughter mentioned going to JTNP that Saturday during their conversation the day/night before.
It seems pretty obvious IMO that Erin said she was going to the park to scout locations for photos/etc. as a cover story because she was meeting CL for a day out.
I also think it's pretty obvious (again IMO) that Erin and CL were having an affair. She told her friend in TN about it. Others on base knew about it (and I don't just think it was backbiting gossip, meaning, people were making up the story out of whole cloth). CL lied about the relationship to LE, which further makes him look as though he has something to cover up.
I don't believe in coincidences either, and I find it very simple to believe that CL and Erin were having an affair, that CL told Erin he would take her out hunting for a day, that Erin concocted the JTNP scouting trip as a cover to go hunting with CL, that Erin was telling her friend about the affair and hunting trip (probably unbeknownst to CL, so the leak about the texts may have been a bit of a surprise to him), and that the two met up off base that Saturday morning...and Erin hasn't, as you said, been heard from since.
Have I missed something? I thought the texts indicated they were going on a day trip together, 2 hours away, to celebrate the pregnancy. I thought it was supposed to be some kind of romantic "surprise". Did Erin also say she was going hunting?