my standard rule is to ignore any IGI death - there are some deaths mentioned in the baptismal reg but they would be nearly always within a few days whilst the initial baptismal entry was fresh in the clerk's mind.
I did some work looking at burials and correlated aginst wills - I gained the distinct impression that many children's deaths went unreported in certain registers.
However doesn't help with Isabella - maybe illeg - was Wm married previously ? - I'm sure that many regs did not always note such