Must a woman who has become pure from menstruation after the 'Isha prayer pray Maghrib and 'Isha combined, and likewise Dhuhr and 'Asr if she becomes pure during the 'Asr time, and is she sinful if she has not done so previously?

You bear no sin for neglecting past prayers due to your ignorance of the ruling. However, you must make up for those prayers you missed in the past to clear your conscience and safeguard your faith. If you cannot ascertain their exact number, then continue to make them up until you are reasonably certain you have performed them all.