Vault-Tec was always supposed to be a dystopian parody of the excesses of capitalism, Tim Cain has said as much and it's just plainly obvious.
Then neither you or Tim Cain understand what capitalism is. You don't get to call it "capitalism" just because you don't like it particularly when the government takes the money of the population under threat of the loss of liberty, decide they want something produced, decide who gets to produce it, and then gives them a stack of money to do it. There's absolutely nothing capitalism about that.
The idea of their being some fundamental flaw that the ghoul's aware of and takes advantage of is perfectly fine, par for the course. But the problem itself is quite silly.
This I agree with. The T-51b is "made from poly-laminate composite capable of absorbing over 2500 joules of kinetic energy, and is covered with a 10 micron ablative silver coating, for protection against lasers and ionizing radiation". So, what weld would it have on chest plate? You can weld laminate composites, but why do that when you could just form it in that shape? You're not expecting that chest plate to move, you want it to be solid. Not to mention welding those pieces together instead of just forming them in that shape would make the manufacturing process that much more difficult. You don't weld together the interior of a hot tub, and a lot of the modern ones are composite laminates.
I chalk this up to the writers not knowing what Power Armor is made from.
Shady Sands' Fall in 2277 - This one is obvious. If Shady Sands fell, even if not to a nuke, it retcons a lot of dialogue from New Vegas where people were saying the city was fine but suffering from several issues born from the NCR's greed and expansionism.
This is clearly a retcon. That chalkboard is in Vault 4 where the refugees from Shady Sands are. The only reason not to put a date on a timeline would be if it just happened or you don't know the date. Since the refugees there would know what year it was nuked, you can rule that out. Considering Maximus was a Shady Sands survivor as a kid and he's a full grown manlet in the show, it didn't just happen. They obviously intended Shady Sands got nuked in 2277, which is why it says it fell in 2277 and there's an arrow pointing to the mushroom cloud.
It becomes even more clear when Lucy says her mom died in 2277. The only reason why her dad would tell her this is if he thought that's when he killed her. If he didn't think he killed her, she could just pop back in with her PipBoy, right? Since that outbreak apparently also happened in 2277, that's also the only time she could have died and no one question what happened to her. Everyone was locked in their rooms. And how did that outbreak happen? How does a disease get in a sealed vault? If someone went outside and came back, bringing a disease with them - in 2277, which oddly aligns with "The Fall of Shady Sands". That's assuming there even was a disease, and Overseer McLean didn't just say there was a disease to lock everyone down so he could slip out and go get his wife and kids.