هل طلب الزوج من زوجته المسلمة مضاجعة جار غير مسلم لجعله مسلمًا يُعد زنا أم رسالة مقدسة لنشر الإسلام؟

The question is unrealistic and imaginary. It is inconceivable that a sane Muslim would be unaware of the prohibition of zina (fornication/adultery) and that it is one of the gravest sins, let alone consider it a means of inviting people to Islam. Similarly, it is inconceivable that a Muslim or any upright man would not feel protective of his honor, but rather encourage his wife to commit indecency and consider pimping a sacred act.