First-time corned beef cure, long-time hypochondriac

The original point of curing meat in a spiced brine was simply to try to preserve it. The "corn" part of corned beef is referring to the salt crystals, and does not require a specific cut of beef. Before refrigeration, beef had to be heavily salted to help dry and preserve it for transport or keeping. The brine was originally just a way to remove the excess salt, while introducing some other flavors into the mix. So you can use whatever cut of beef you like, and your chuck roast pieces will do just fine. You are not going to poison yourself with the curing salt, so relax. I won't bore you with the technical explanation- suffice to say your meat won't absorb it all out of the solution. It takes several days for the brine to even penetrate the full thickness of the meat- which is why the long soak process. If meat isn't soaked long enough, when you cook it, you will notice that the nice pink color you were expecting doesn't quite get all the way to the center of the meat, and it will look just like it always does. If your cure is complete- that is it penetrates the full thickness, then you get the nice pink color all thorough the cooked piece. Still tastes good, either way- even if you get a small uncured strip in the center.
When it comes time to cook your meat, I recommend you do so the way you would ordinarily do a chuck roast. It would like some braising liquid, even in the oven, though. Personally, I don't care to boil corned meats either- even on the stovetop I do a braise. No point in having a pot full of flavor cooked out of your meat that you end up pouring down the drain.

So relax a bit. I was nervous the first time I cured a cut of beef on my own too. But you know, as a rule- meat you cure yourself most always tastes better than what you buy ready done. Modern methods inject the cure, rather than allowing time to do the job. It makes the process faster, not better. Your way is going to work out better in the end. So hang in there, and be rewarded in a couple days' time.

I don’t think you have anything to worry about. The amount of the Prague’s Powder sounds right given the amount of meat you have. As for the cut of meat, that should also be ok.
I also would not worry about toxicity. Meat is not a sponge which is why the curing process takes almost a week. It takes time to penetrate muscle which is primarily what meat is. Given your thinner cuts if chuck, I think your corned beef might be ready now, though I don’t think a few extra days is going to do any harm.

When you are ready, take out your meat. Rinse off the curing solution and pay dry since you will be baking it.

Btw, I am very interested in how you will be baking it. I prefer baking to boiling. Texture is better and it’s easier to slice.

Good luck! Let us know how it went :)