I co slept on a 100% latex mattress. It was just one giant heavy slab of latex. It was very comfy and I never worried about DSs. One slept on his back, the other was a side sleeper like you descibe.

I now sleep on a new mattress which has a top memory gel layer. I wouldn't be concerned about co sleeping on that either if only I could steal a baby from someone.

Ive never had a Tempur one bit I have heard that they are meant to be bliss. How much of a layer is memory foam type? I am guessing that the mattress only sinks depending on the weight applied, so if your DS was on his own, he'd be fine. Is the concern with the pressure you apply to the mattress and DS being right next to you?