Muslims participating in memorial services

In order to appease the kuffar, Muslims are today participating in memorial services, joint prayers with non-Muslims, vigils etc to remember the those (predominantly non-Muslim) that died in unjustified terrorist attacks. I am not a scholar but I even see scholars condoning participation in such events by participating themselves. Am I correct in understanding that participating in such prayers, vigils and services is haraam because we are not permitted to pray for a dead kafir and also imitation of kuffar methodology in expression of grief is not permissible. I also find it discomforting that at these events there is free mixing and acknowledgement by virtue of participation that all religions are equal. Would you please provide proof in light of Qur'an and Hadith whether participation in joint vigils, memorial services and prayers is permissible as family and friends do not just accept a simple answer of no without evidence of why?