Whatever bag was in the truck that day" is probably closer to the truth than you'd guess, but I'd push back on the idea that it was pure laziness. Back then mortar got mixed on site in small batches, and color varied a lot by sand source, so even the same mix could look different from one day to the next. Sloppy joints bug me too, but some of those old repairs were done fast because the wall needed to get closed up before weather hit. My take is the speed was real but the color mess was more about materials than guys not caring.

Ha, that's a wild way to put it. Honestly subs are just captions with extra steps when you think about it.

Your sister's advice is setting you up to fail. Silicone doesn't suddenly behave at 375, and a mat that warps on the first batch is just a bad mat. The temp limit is a legal number on the box, not a magic line where cheap stuff works right. You paid outlet mall prices for outlet mall quality, that's the whole game there. Pricier brands do hold up because the silicone is thicker and the fiberglass core is actually woven in straight. Going back to parchment is fine but it's you giving up on the thing working, not the thing being impossible to make right.