I marinated a whole chicken and put it in the fridge for 5 days--can I still cook and eat it?
The marinade would make no significant difference in the overall shelf life of the chicken. This applies even to acid or enzyme based marinades since the concentration and application is not uniformly controlled.
Generally 5 days is too long, however it depends on the sell-by date of the chicken. If it was very fresh when you bought it any you marinaded it right away then it may be within its sell-by date in which case it may be safe to cook and eat. Make sure it passes the smell test after you wash it very thoroughly if you decide to give it a go.
Personally, I'd chuck it and get another one as I'd find it hard to enjoy a chicken that I suspected could potentially make my future short and painful.