n if the orphaned neville grew to be brave n confident, he might hv had a chance with voldemort. if he was the same neville we see in hp books, he'd hv needed some time to prepare for it...
again, he can't either be this or that... he'd hv had many shades like harry n some similar qualities being orphan. (harry as we read in the books- an orphan- not with parents...) so, everything depends on neville's character!
here, snape definitely would remain a death eater till the end... he'd be sent to azkaban... or even if he faked being imperiused, he'd remain evil...
in the 2nd scenario,
i believe voldemort would hv chosen harry and snape would hv been a death eater till the end...OR if voldemort went behind harry later, harry's parents would still be there shielding him... voldemort would hv to finish the whole family and may be, snape'd be confused abt his role n MAY choose the good side again, having seen lily's death... here, everything depends on voldemort's choice... (which would be harry, according to me!)