I believe that if I have one cross to bear in life, made doubly burdensome by my own devotion and interest in our beloved saints, it's this endless argument:
I can only speak for myself, but as a protestant, the reason I reject asking those in heaven to pray for me (and certainly reject praying TO them) is that I believe there is one mediator between God and Man: Jesus (1 Tim 2:5). Further, God does not answer prayers based on WHO is praying/asking, but based on praying according to HIS WILL. I believe we can confidently come before God with our prayers, knowing that HE hears us (1 John 5:14-15).
Further, I do not find any biblical reference suggesting we ask saints/those in heaven to pray for us. Are there? All biblical examples I have of people "speaking" to those who have died are in the context of witchcraft/sorcery/divination/necromancy.(1 Sam. 28, Lev. 20:27, Deut. 18:10-13)
While I believe those in heaven are aware of what is happening on Earth, I see no biblical evidence that they can necessarily "hear" us. While they are in heaven, they are still finite beings, not omniscient.
I do think there is a general misunderstanding of Catholic doctrines in the protestant church, and certainly, among my Catholic friends, I have heard them say they are "praying TO St. XYZ" or praying a novena TO so-and-so, which clearly doesn't help the understanding of the actual Church doctrine.
My face is permanently blue trying to explain. I will never give up. I will eventually expire, and when I do, I will resemble Papa Smurf.
So let's take your objection a step at a time. "There is but one mediator between God and man: Jesus." Only God can answer our prayers. Jesus is God. Did that slip your mind? When you pray to Jesus you are praying to God.
But going along with your premise, do you never ask people around you to pray for you? I can't believe that you don't. You are asking them to pray to Jesus, God, the Holy Trinity, for you.
In the Catholic Church we believe that those who are with Jesus in Heaven are just as active in their prayers for us as is your or daughter or butcher or preacher or whoever it is that you are asking to pray for you.
That's why all the prayers asking for intercession end in "Pray for us". I can't picture a Heaven where the Church Triumphant (which is what we call those people who have "arrived") are just running around, jumping and skipping and sticking their fingers in their ears and humming. God cares about us here on Earth. Why would anyone think that people who are now with God don't care. After all, one makes it to Heaven by being in harmony with God. And they are no longer finite beings, but have life everlasting. Hooray for them! I hope they are praying for me. I can't think of anyone better to ask to pray for me, than people who have won the battle that I am still fighting, so to speak.
I generally don't get into "Bible fights". But here are a bunch of Bible moments in which the early church talks about praying for each other.
“I beg you brothers, for the sake of our Lord Jesus Christ and the love of the Spirit, join me in the struggle by your prayers to God on my behalf” (Romans 15:30). “But you must help us with your prayers, so that on our behalf God may be thanked for the gift granted us through the prayers of so many” (2 Corinthians 1:11). Other examples of intercessory prayer for other Christians may be found in Ephesians 6:18-20; Philippians 1:19; Colossians 4:24; 1 Thessalonians 5:25; 2 Thessalonians 1:11; 3:1-2; Philemon 22; James 5:13-16, and 1 John 5:16.
We believe that the saints in Heaven are as much a part of our Church as the guy in the pew next to us. But....this is a very important "but" about your Bible quotes....we strongly discourage the Church Militant (the arm of the Church here on earth) from asking for the intercession of the Church Triumphant unless they know for a fact that the person they are asking to pray for them is in Heaven. Which is why we canonize saints. Heavenly proof of Heavenly Triumph. We don't want you asking Uncle Bob to pray for you. We love Uncle Bob, but only God knows what happened to him. We hope the best. But the devil is tricky. You may think it's Uncle Bob, but it's really Uncle BeelzeBob.
We do pay attention to Scripture.
I certainly hear many, many Catholics saying they are praying TO a saint. They actually may misunderstand the doctrine themselves. But most of them do understand. It's just Catholic shorthand for what's really going on. Like when you say you're going to "make dinner", when actually, you have to boil water, wash meat, cut up vegetables and try not to burn yourself, put on the pasta, put the meat in the oven, throw the vegetables in the microwave.....
Our real difference seems to lie only in the disagreement of what goes on in Heaven. I believe that those who in harmony with God, care for us as He does.