Congratulations on finishing your doctorate.
Your photos show good work. And you did a good job on the save with the new tang.
A few southern schools used that arrangement with the guard holding the trigger plate. Some of the antiques show failure there, with the guard bent down and the trigger plate coming out of its mortise. I've seen a few of those. There's a North Carolina rifle by one of the Whitsons that it looks to me like it has a repair based on that kind of problem. Looks like a tang bolt was added later, put through the trigger plate. But that doesn't mean they always had issues, either. That style was used by at least one old maker in the county where I grew up. I've seen antiques built that way that are still perfectly fine, with the trigger plates still tight.
It is something to be avoided? I don't know. I know several people who think it's a cool detail. I do think there are a few things from the old days that we want to avoid. Breach plugs with only three poorly fitted coarse threads, for example. But it seems to me that some other details, like this one, might just be part of the charm of a particular school.
So I guess what I'm saying is that it really depends on your philosophy. Some of the guys on here are functionalists. Others are traditionalists. Some like every detail perfected and perfectly machined. Other want to see stuff built like it was in the old days, true to design, even with scraper marks and tool marks everywhere the old ones had them. There's plenty of room for opinion, and opinions can differ widely. So I hope you get some good feedback, but don't be surprised if the opinions differ.
If you want to keep a balance between improving function and traditional design and appearance, then, like JBulitz said, it might be possible to use a bolt and hide it. Hiding it under the trigger guard, as he suggests, is an option. Another you might consider is adding a threaded lug to the hidden side of the trigger plate. The tang bolt would thread into the lug, rather than passing all the way through. The head could be made to look just like the wood screw(s) used on other parts of the tang. The lug could be silver soldered on, or dovetailed into the plate. That would give you strength without affecting the asthetics of that detail. (If I was gonna use a lug, I think I'd use the dovetail option, or maybe use a loose dovetail until it was all constructed and threaded, and then glue it together with silver solder. Anway, I can imagine doing that without having to have a bottoming tap. I really hate having to grind the tips off my taps.)
You could just drill the trigger plate and tap it, too. You might be right in line with Whitson if you do.