Hi JT.
No. The last part of your reply is not correct. Or I can say it is correct if there were no actual wind which is the movement of air mass where the plane is flying. But in reallife this is not possible, always there is wind.
Yes The IRS calculates velocity vector (so the position) as you explained, but reference is geographical position, in another word, earth, not airmass. So, as in your explanation, if they are different (heading and track), algebrical subtracting true heading from true track may give you an idea about where the air mass (Relative wind) is hitting the plane.
But this is not the case, the air mass where we are in also moving.
Consider a boat is crossing the river with 90 degrees. Relative to bank airplane is moving sideway (similiar to IRS calcualation); but relative to river airplane is moving straight, no sideslip. Water is hitting from nose.