Is it permissible to supplicate collectively after the marriage contract, and is it permissible for those present to say "Amen" and raise their hands, audibly or silently, with a request for evidence?

There is no objection to supplicating during the marriage contract for the couple and others. It is commendable to supplicate for blessings for them after the contract, based on the Prophet's (peace be upon him) saying: "May Allah bless you, and may He bless upon you, and may He unite you both in good." It is permissible to raise hands and say 'Ameen' to the supplication, and to say it aloud or silently. However, collective supplication should not be taken as a customary practice or Sunnah (definition) in this context.