Is being surprised or smiling at the Imam's recitation of short verses after Al-Fatiha considered mocking the religion, and does this entail falling into disbelief or affecting the marriage?

If the person smiling does not intend to mock God's verses, then their action does not fall under disbelief. However, ridiculing the lesser deeds of others is forbidden by His (the Most High) saying: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them." Allah may accept the prayer of one who recites a short verse with sincerity, and reject the prayer of one who recites a long surah with conceit or hypocrisy. It is a Sunnah for the Imam to shorten the prayer, and whoever seeks forgiveness and repents, it is hoped that Allah will forgive them.