Gino - I see that you've posed several questions here:
1. Should a customer tolerate rust and a faulty idle screw (not clear what that means) in what was portrayed as a rebuild? No, absolutely not. The rebuilding process assumes a thorough cleaning, derusting and replacement of parts as necessary. And if you have significant rust visible, who knows what the condition of the internal air passages is. Don't feel bad about returning it. But if you've gotten two defective rebuilds back to back perhaps it's time to get another supplier.
2. Can you tolerate a bit of rust in the bowl and other internals? Yes. Tom Endy's article that Old Ugly referred to is a good place to start to get perspective.
3. Can you remove (or a better term, convert) rust? Yes. I've had good luck using Evaporust on rusty carb bodies. You need to completely strip the casting, let it soak thoroughly, scrub, possibly repeat a few times (rust penetrating into a rough casting will not always convert the first time) and then reassemble. But at that point you are doing the rebuild yourself, which is what you paid your supplier to do.
Doing a full rebuild on a Zenith can take a couple of hours and parts, especially labor if you include painting with fuel-proof paint. Figure in the cost of labor and parts and if you think you're getting a great bargain, it's probably too good to be true. The regular reputable vendors charge on the order of ~$300 for a rebuild, so use that as a comparison.
Good luck.
JayJay