We have not come across any hadith which says that whoever makes someone get up from his seat will be deprived of his seat in Paradise! But whoever makes someone get up from a permissible place that he reached first is wronging him, and he should ask him to forgive him and not expose himself to retaliation on the Day of Resurrection when the issue will be settled with hasanaat (good deeds) and sayyi’aat (bad deeds).

There is nothing wrong with responding when a non-Muslim greets a Muslim at Eid or on any other occasion, by wishing him well and praying for him in an appropriate manner that is equivalent to his greeting. So one may say to him, “May Allah help you” or “May Allah honour you” and the like, and if the intention is to pray that Allah bless him and guide him to Islam, then that is more appropriate and better. And Allah knows best.