The provided LR tongue looks way to light for loads up to 3500kg. It looks very similar to my Ford one that is only rated to 1600kg. On the LR tongue the square section tube is actually thinner than the 1600kg Ford tongue. The section where the tow ball mounts is a little thicker on the LR tongue.
I am using my Ford WDH tongue (not using the spring bars) in the LR receiver. This tongue is rated at 3500kg.
My next move is to buy an adjustable heavy duty tongue so that I can sell the Ford kit.
I don't think using different tongues will affect warranty. But you should ask LR Australia that question.

