I think you raise a legitimate issue. I have pondered this belief as well.
I don't think, by attributing the ability to intercede, we are attributing omnipresence. It doesn't necessarily follow. Omnipresence would encompass much more than the ability to hear prayers. Being able to hear someone's prayer request seems, to me anyway, a small and limited facet of what omnipresence entails.
I also wonder if it is fitting to apply our concepts of reality to the the after-life. I mean, our constructs just may not apply at all. We can conjecture and speculate, but our knowlege may be too limiting. I think it helpful to consider the shock an infant experiences when it emerges from its mother's watery womb into the world of light and air. So it may be for us. When we leave this world, our destination and its conditions may very well be utterly inconceivable. The laws and conditions that govern our world and this earthly experience just may not be applicable in heaven.
And yet we have all this antecdotal evidence that there is something to this intercessory prayer stuff. Centuries of it. In my own case, I had been looking for a job for a few months, a particular kind of job with specific conditions that would not interfere with other things I needed to do with my life at this time. It was a tall order and very unrealistic. My financial situation was becoming desperate when I happened upon a link that a Freeper had provided to "Mary, Undoer of Knots." I felt I had a knot that needed to be undone, so I offered a prayer to Our Lady under that title. A few days later I had landed a job that met my precise conditions and has surpassed my expectations. The knot was undone.
Naturally, I'm tempted to write it off as dumb luck, coincidence, and ponder the inevitable questions. Does God care where I work? Does Mary really have anything to do with it? Or by being skeptical am I limiting God's love? Am I applying my incomplete knowledge to things I am incapable of conceiving?
Anyway, it makes interesting food for thought.
People may misinterpret our great love for our wonderfull spiritual mother, I almost understand that. They don't realize that love is infinite. The more we love, the more we love God. Insulting God's children is, to me, the enemy's influence.