We stayed in Pentewan last year. There is a small bike hire place at the start of the gravel road, they sell maps of the Clay Trails, there is a whole network of trails that can take you from the south coast to the north. They are not the most challenging trails in the world, I rode them on my gravel bike rather than MTB but plenty of options and you can get around avoiding the traffic.
Bike parking at Eden Project wasn’t great considering how green it is supposed to be. Lost Gardens of Helligan is good, accessed completely off road from Pentewan.