Am I justified in my frustration?

Perhaps let the internals soak in White Vinegar as I believe the axle spline is rusted to the side gear.....

Then use two impact guns at the same time to try and break the bond....

One impact gun on the pinion nut.
One impact gun on your slide hammer adapter.
Alternating from forward to reverse on both guns providing “Rotational Shock” to try and free her up....
That sounds like an approach I may take.