If you’re talking about sudden infant death, I don’t think it’s related to germs, although it’s a poorly understood phenomenon.
However, in recent years there has been a backlash to the backlash, and under certain conditions (no drugs/alcohol, exclusive breastfeeding, no blankets) some doctors will recommend co-sleeping. It’s controversial.
SIDS isn’t a cause, SIDS is a name for when no cause is known.
Ultimately risk falls off around four months though, so yeah maybe 2.5 months is a bit early to try weird things.
Basically the SIDS risk as understood currently is like you said: obstruction getting in the way, baby not waking...bad outcomes. Co-sleeping with the parents increase risk by bringing pillows, blankets, themselves into the bed to add obstacles or opportunities for the infant to suffocate.
It's a highly nuanced topic. In my parent's day it was common for babies to be put to sleep on their stomach whereas now the advise is to back sleep even though there's increased risk for flat head, but lower risk for SIDS. Once the kid is able to turn themselves it becomes much more simple as they'll move around throughout the night so back/stomach becomes a non-issue once they reach that age.
I'm not saying this is what happens in every instance, but it's probably the way it happens at least some of the time.
you mention giving your chewed up food: I read a few years ago that babies who sleep in the same bed as the parents have a much higher mortality rate; assuming it's true, I guess that with mouth to mouth germ exchange it would be even worse.
Also, a lot of food today is full of chemical residues etc that are tolerated by adults, not quite so by babies.