Try putting your auto dark helmet in the direct sunlight for 4 to 6 hours to recharge the solar cell. Apparently some helmets say to do this in the manual. If it also uses a battery, it may need a new one too.
Your log cart looks pretty heavy for the size of the axles but I think you're pricing it too low for the amount of time and work it takes to build even if it went perfect. I agree with Dick2 that you should make a good drawing and tack everything together first before welding it up. It's easy to grind a few tacks out. Are you using a gouging tip in your plasma? Sometimes it's a lot better to just use a cutting disc in a grinder than to gouge a weld out. Why are your MIG welds getting porosity? Do you have a fan or heater blowing?
It's nice to build things yourself and you can get a lot satisfaction out of it but when you're spending several hundred dollars to make something that you don't need yourself and are hoping someone will buy, it gets pretty expensive. When filming the welding, you should experiment with a #9 or 10 lens taped on the camera so you can actually see the weld the same as the person welding. Otherwise it looks like just a big bright light. Maybe the camera needs to be closer or more focused, I'm not sure.