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.
Yes, but its track is significantly different from the 90 deg heading. The current is the same as the wind. Assuming constant current, if the heading is changed, the track will also change, and the calculation of the current will remain constant.
However, if a small amount of rudder angle is applied to the boat, the boat will turn and the track will continuously change. An IRS would sense the rotational acceleration (maybe you forgot to give credit for this?) as well as the heading change. If sensitive enough, it would also calculate an instantaneous slip angle as the boat turns, based on the previous calculation of the current.
If the current velocity changes, it would also be detected by a sudden sideways acceleration. Transient calculations would be iterative until a steady state is achieved.
No need to try, You are right, yes it turns,
Well, therotically it should not turn. All the documents and force vector diagrams say so, google it.
Please post a couple of the URLs that you found to support that theory. I suspect you are oversimplifying the vector math.