Do all children with strokes at birth have lifelong seizures?

No . No but not uncommon based on location of insult and size .
No. The impact of a stroke varies considerably with some developing seizures and some not. In the early years of cat scans they scanned hundreds of normal infants to begin understanding what the study showed.They found a small percentage of "normal" infants had small areas of tissue injury consistent with a bleed. The general rule is: the worse the initial injury the more likely to have problems.
No. Unfortunately this is too difficult a question to answer more in depth without more information. There are too many variables to give a universal number. A visit with a pediatric neurologist is required to fully answer your question.