This is my take on the theory of that call as you explained it.. having nothing to with what you saw or called.. some times with OBS (and other calls) you subconsciously see things that trigger the call that are hard to explain.. so I think your judgment stands for your call..
But (for the sake of an OBS convo, I'll not worry about the crash for this moment)
My take is - this is an out.
You've got F2 set up a couple up of steps and the runner not deviating (so not impeding) but another step our two out from the catch.
Yep she can't slide too far.. and the other things you pointed out..
Thats how you get an out. Shes not only out.. but shes out by 5 or 6 feet.
OBS is not a punitive punishment meted out based on how you think a particular play should be made.. it is impeding the runner without the ball (and the other things of course). If that were the case.. punish the 3b coach for sending a runner who was going to be out by 5 feet.
Anyway WMB, I dont particularly agree with your premise. This sounds like a heads up play by a catcher. Putting it another way ..
The runners "entitlement" to unimpeded access ended the moment that catcher caught the ball. If she was at no time before that exact moment in time impeded in her progress.. there is no OBS.