Its not probable. Its not possible. If you think of it logically, its not possible for Sephiroth to survive the blast either.

His shield can defend against anything except what he's using right? He could wait for Magus to use Shadow magic and then cast Shadow Flare. This attack is really strong. The second most powerful Enemy Skill except for Pandora's Box. It could probably do some serious damage.

Sephiroth's Wall could help protect him against Magus's attacks.

Materia SHOULD be allowed. Sephiroth can only used the materia given to him in the flashback. He can't switch or get new materia.