My little guy is fifteen months old, and this same exact thing has been going on for a while. Also, at this age, they hate to stop what they are doing, even if they are tired, because they are learning so much, just got a bit of a taste of some control over their bodies and they don't want to quit. He is also in need of having either myself or my husband within view or reach at all times.
I think you are right on with your assessment of the situation, and right on track with what you are doing, and it will pass. With mine, as long as I manage to sneak out without him seeing me leave, it's all good. I'll hear him right after I get out the door, and he'll just whine once and go back to sleep. If he "catches" me, though, I usually have to pat him a bit more, then try to sneak out again, or he wakes himself all back up again by standing up and crying. I know where all the creaking floor boards are in the room, and it's an art to get out.
That being said, at the same time, when my little guy has an ear infection, putting him down to sleep is hard because the prone position increases pain. Is he eating? Ran a fever recently? Pulling at his ears or real crabby at any other time? They should come with more instructions. Some kids don't give much in the way of outward signs that they have an infection.
So hard to know what to do sometimes, isn't it?