That's a broken weld, not caused by rats. If the cage is stable I wouldn't worry mych. I had similar breakage on my first, used FN. You can get brackets from Lowe's, etc, but you'll need a drill and some screws and nuts to fix it. When I put out FN182 together it did seem like it took more force than should be necessary to drive in the wheel mounts.
This is a Feisty Ferret, not a FN, unfortunately. Big part of why I'm so angry about it all. This whole cage is just single-point welds. And the broken bar was hidden at first by it being glued on, which is a bit scummy in my opinion.
The wheels are hard to get in/out. I had a wheel break on my FN and it was super hard even for Shane to get the old one our and put a new one in - definitely took some elbow grease. The cage really doesn't look too bad from the pics you posted.
Base meaning the tray. The ramps aren't a big deal at all, I don't plan on using them for long anyway. But the base has significant dips where things could leak through easily. The base tray thing is above where the bar is supposed to be, since the bottom of the cage isn't stable enough to hold it where its supposed to be.