Dutch Uncle
* Tertia Optio * Defend the Constitution
There seems to be some evidence of intentional burial of Neaderthals. That is true.
The elaborate, ritualistic burial with flowers hypothesis seems to be on shaky ground. It sure is a sexy and edgy hypothesis that has a good chance of publication in high impact journals.
I don't think that physical burial in and of itself proves complex religious beliefs and practices. There are alternative explanations for burial, and other sentient animals have elaborate grieving practices and seem to revere the bodies of the deceased relatives (aka, elephants).
A conservative scientific posture would require supplementary evidence of ritualistic practices, like clay sculptures of fertility goddesses, paintings of animal spirits, or other tangible evidence of ritualistic practices.
Burial being the defining line between animals and humans for me. As you pointed out, even human cultures vary in their burial practices, including cremation.
Human religious practices are so widely different that it would be difficult to define the practices of extinct species of human beings.
Artifacts would be evidence of religion but it's also an aspect of technology. Modern Humans and Neanderthals are clearly different in their approach to technology.