peterjack ji
My problem with the question, "What is the truth behind claims that sikhism was influenced by hinduism and islam," lies in the semantics of the question itself.
What is meant by the word "influence?" Guru Nanak was no stranger to either religion, Hindu or Muslim. Some would like to argue that he took a little of this and a little of that and put it together to create "Sikhism." Careful reading of Sri Guru Granth Sahib, coupled with some knowledge of the biography of Guru Nanak's life and interactions with Hindus and Muslims. tells a different story. It tells the story of someone who refuses to judge or be scornful, but at the same time is pointing to the moral hypocrisy of adherents of those faiths in his day. Every shabad where he refers to Hindu devtas, concepts and practices, and the shabads where he refers to Islam, can be aligned with his travels. In each shabad he is explaining how there is a higher moral and spiritual calling that contemporary religion has ignored. That moral and spiritual truth is what he taught through the shabads.
So "influences?" In my opinion he was reacting to influences and suggesting we rise above those influences and take a different path.
In response to your second question: there is considerable scholarship that suggests that Guru Nanak had laid down the foundations of a "religion" during his lifetime. But let us not over-define the word "religion." What do you mean by "religion?" Guru Nanak had, by the time he founded Kartarpur, afollowing organized around a spiritual practice that was clearly distinct from other contemporary religious paths. He also had a following throughout India. He preached the importance of living the shabad guru, and the message of devotion, honesty and hard work. That may or may not describe a religion according to one's definition.