Variation in materials can make a design work or not work sadly - if you’re designing yourself you can measure your material to be sure, but using someone else’s design sometimes runs that risk.
Now, if you were able to get the slots together, just not all the way, then you might try hitting the flat side with a mallet. If you don’t have a rubber one you can use a hammer, but you’ll want to put another piece of wood between your finished piece and where the hammer is hitting cuz it’ll probably get dented!
It’s right up there in the first post, under where it says “here’s the file.” Right click on the design and you should be able to “save image as” to save the .svg file to your computer.
When I download to svg, the image is like 1.25x, do you just resize the whole set of images to match the 1" square. I would just do that but I am actually helping a family member at home, and I am away from my GF.
You’re probably getting bit by the 72 dpi vs. 96 dpi issue. My design is sized at 96 dpi to match what the Glowforge expects, so if you set your design program to that, everything should be sized correctly.