I think I'll have to ponder this one for a while! Obviously we don't want to increase any risks, but as you say if 20 seconds is sadly too late I'm not sure what would naturally wake either of us up any faster!
(Believe it or not I'm also not trying to get neurotically paranoid about everything!)
Certainly looking at the SIDS findings, having the baby in with you in the early days does seem to reduce the number of deaths, though, as with all of the changes to reduce cot deaths, they're not 100% sure why - the theory is that hearing you breathe reminds the baby to if they stop for any reason. The risk begins to reduce at six months, and drops sharply again around the one year mark.
In my case, keeping the child in with us until at least six months was the one SIDS prevention recommendation we didn't adhere to. We had small child in with us until they outgrew the Moses basket, at about eight weeks. I had been given an Angelcare monitor so what we did was reverse it - in my very, very sleep deprived mind this then covered the 'need to hear someone breathing' recommendation. We used the reversed monitor until the six month mark.
Unfortunately, if it's a proper SIDS case then by the time the movement monitor goes off, and you get into the room it is likely to be too late to save the baby. I never used the movement alarm due to this.
Once you are aware of the risks and comfortable with the potential consequences then it is your call to make on how comfortable you are with which SIDS prevention recommendations you go with and which ones you don't.
Also, on a cheerier note - congratulations and best of luck with the impending arrival.