Well it was a stressful day but the van is back on all 4 wheels, sitting in my driveway, and (another) crisis appears to have been averted!

After taking yesterday to reflect and plan, I got up bright and early this morning, had a good breakfast, and drove over to my parents' house to get started on the splines. I used a hand file and a small Dremel with the thinnest wheel I could find. I started by removing the material on the side of the teeth that had dents on it, which indicated to me that they were the splines interfering with the inner teeth of the differential and keeping the shaft from going in any further. I used a hand file to remove any excess material that would have messed with the diameter of the shaft.

After a round of this, I was able to get the shaft to go in about 10 - 15mm before getting hung up again, progress! I pulled the axle, looked to see where down the splines it was getting caught up now, smoothed it out with the Dremel, and tried again. This time, I got it to the point where the bearing was up against the axle housing. With the finish line in sight, I gave it a few quick final zips with the Dremel, got the backing plate and drum assembly hanging off of the studs, and got the shaft back into position.

My dad and I said a little prayer to the van gods, I grabbed the rubber mallet, yelled the battle cry of my generation, which I guess is "send it" now (What was wrong with YOLO?), and the shaft slid the rest of the way in after two generous love-taps. A wave of relief washed over me and my dad, although I was still worried about the shaft possibly being bent.

After reassembling the drums, torquing everything down, bleeding the brakes, and adding fresh diff fluid it was time for the test drive. The van started right up like nothing ever happened and drove down the street as smooth and as straight as it ever did, no wobbles or vibration to indicate a bent shaft. The noise I was chasing back there appears to be gone, but there is a slight "ssshhhh" sound coming from the rear driver side wheel. You can really only hear it if you're back there, the old noise was pretty audible from anywhere in the van when in motion. I noticed that most of my wheel weights are missing on that wheel so maybe that has something to do with this slight noise? Oh well, that's for another day I'm just going to enjoy this victory for a minute before I dive back in.

As far as the shop that did this goes, I'm going to explain my experience in a Google review, never go there again, and leave it at that. Thinking about it, the botched job itself doesn't bother me as much as how they took my money upon delivery, didn't mention anything, and let me walk out of the store with a part they had to have known they messed up. I'm walking proof that sh*t happens, but I always try to own my mistakes. To just send a customer on his way without disclosing that was what made me decide to find another shop in the future.

TL;DR
If your axle splines get goofed up like mine did, a Dremel with the thinnest wheel you can find, a good hand file, and patience are your friends. Remove as little material as possible, check to make sure that not only are the spline teeth spaced properly, but that their diameter is equal to the other non-goofed splines. If you're going to have a shop remove and press new bearings, take before photos and closely inspect the splines prior to leaving the shop.

Thanks again BB and Tim, hopefully, no one else here will have to go through something like this.