Yes, the file name is what caused lots of my problems.

I named it JUNK because it was a copy of the original and would remind me it's a useless copy (unless I edited enough to be what I wanted).
You are correct about fillets being tricky. The inside of the U can't have fillets as it needs to have 90 degree bends, but thought to pretty the outside. I assumed, and it makes sense, that trying to add a fillet that imposes on something will cause issues. In some cases, I really don't understand why the fillet fails because nothing seems in the way. All I know is I add a larger number, not by much, and next minute I get either an error, the model vanishes due to an error, and/or I get a red explanation mark next to the name in the tree.
It seems enough room exists to add fillets to 'Sketch', but it just gave me errors. As mentioned, I thought it was the fillets I added when I clicked on a face and added a fillet. Since I didn't really understand, I just clicked a face, added a fillet, and looked to see if it was what I wanted. Other times I'll add some large number just to see what/where got affected, but large numbers results in the model vanishing on me.

At least you confirmed that the model can "break" easy when trying to tinker with stuff at the end like I did. I spent quite a bit of time trying to figure out why deleting something I added caused errors. No matter what I did resulted in the model being junk; unless I deleted everything and started from 'sketch'.
As for exporting the STL, you have mentioned the benefit of exporting individual sections in the past. I don't remember which item(s) from the tree I selected. Initially I tried CTRL-A, also 'body', and the last in the tree.
I assumed selecting "Pad004" would just export the one Pad rather than all the previous stuff. So it's always either the last in the list (in this case Pad004) or 'body'; unless I want an individual piece?