Is this my mind playing tricks on me or is this part of the whole MLC journey?
No, your mind is NOT playing tricks on you, and the "children" are but a part of the MLC journey. They are the children of his issues, it is my understanding that something happened to them around those ages, and that is why they show up to speak their piece.
It was also my understanding that the children generally show up in the Acceptance stage; that is how I'd seen it when it was happening to my husband years ago.
BUT-the stages can mix up and not go in any one order; as I've seen from reading different people's posts.
From what I'm reading on you, your husband is still firmly planted in Replay; the OW is not gone, and the "seeds" of different stages are within him. He is having a terrible time with this; and you're right to continue to let him go, and get on with your life; dealing with no more than you have to in the way of his drama.
When/if he gets into the actual acceptance stage, they may show once again; even if they back away before then.
I'm trying to remember if I saw any children before the Acceptance stage, and the only thing I can come up with in memory; is that the problems my husband had outbursts/porn problems/OW were evidence of teenage behavior; and that was VERY evident during Replay. I remember it fading as he got rid of the OW, dumped the porn, suffered the withdrawal from her, and came forward into Depression then Withdrawal. That took time to complete. The second child I didn't see until Acceptance.
When he headed into Acceptance, I saw the break/disintegration, and it was two children I saw. The outbursts didn't come that time, just the children. The most my husband did in that stage as the teenager was complain and loudly. The other one was 4 years old, and loved affection, the more he could get, the better.
I can understand you being startled by them..you never know what you're going to be facing from one day to the next.
Do not know if this will help, but I thought I'd at least tell you that you're not losing your mind; what you're seeing is not a trick of the mind. This is real, not only to you, but to him. He seems to be in a place before you met him, and if you look at him, you'll see he doesn't "know" you..and that is really startling. I've seen that, too.
Have a good one.
Remember, as each person is different, every MLC/Transition is different..what works for one may not work for another. Most of the time it is trial and error for ALL involved.