What is wrong with asking Mary to pray to Jesus too? I may be a little tongue-in-cheek here but, her prayers probably carry more weight than most peoples’ prayers. Rallying her to your cause surely cannot hurt.
More seriously- Maccabees provides an excellent example of how praying for others can be useful. But that isn’t in your bible so...
1 Timothy 2:1 ESV
First of all, then, I urge that supplications, prayers, intercessions, and thanksgivings be made for all people,
The bible seems to imply that praying for others is useful.
If I am strong in my faith, and my prayers come from an agape form of love and mindset, Christ will provide as long as we are willing to receive it. Of course God's way of answering may not be what I expect, but it will be there.
When Jesus taught us to pray, He didn't say "Our Father And Saints". He simply said "Our Father". There's a reason for that.
Prayer to God, and God alone, is sufficient. Timothy 2:1 is quite right - praying FOR others is very useful and we are commanded to do so. Praying TO others who are not God is not necessary.