Well, when you are being interrogated about a crime, the first instinct is to defend yourself, whether your are innocent or guilty, but especially if you are guilty: you are on the defensive mood, you are on guard, your instinct is to deny any involvement, so you say, I was not there, I know nothing about, etc.. It's hard to imagine at one point you suddenly have a 180 degree reversal of the defensive mood and are actuallying thinking about the crime/murder and give a truthful statement, I was there, or I was definitely the last person to see him alive---why, of course, I killed him!
There can be a number of reason he made that ridiculous statement. Need to know the complete context, and what he and JM talked about. E.g. it could be that JM told him where he's going after the meeting and what he's going to do, which led him to believe JM would not be meeting other people. Again it's hard to say. But to me it's no big deal that he made that statement. So-called Freudian slip hardly applies for reason given above.
The same about using past tense. In fact the defense lawyer had made good explanation: the police interrogating CM was using past tense in asking him about JM, so naturally CM replied in past tense. Also, given that he didn't know what happened to JM, there's no reason he should use present tense to talk about what JM had been like. Without the complete interrogation content, let's say the police asked, "what was JM's daily routine?" Then it would be natural for CM to reply, "He was always up at 7 am, then fed the dogs. He loved those dogs.... " He didn't know where JM was and what he was doing during the disappearance, so he had no reason to say, "He is always up 7 am, then feeds the dogs. He loves the dogs...."