My question is still, "what's the support and reasoning for praying to the saints who have left this earth?"
The reason has been well established and understood; it's the evidence to support the practice that's lacking.
So far, all I've read is:
1. There's very strong Biblical support for asking other Christians to pray for you in an intercessory manner.
2. All Christians are saints.
3. All the saints are part of the body - the church, even after death.
Well, really, it can end there. The last 2 are, to varying degrees, irrelevant to your question, then. Do you doubt the legitimacy of those 3 points?