1. Unlikely but possible. Perhaps you accidentally dislodged something in removing the air cleaner without realizing it (or partially dislodged it and it vibrated loose from there).
2. I guess theoretically possible, but extremely unlikely in my opinion. What possible motive would a repair place have for surreptitiously replacing the generator with a similar or identical one? How long do you suspect a repair shop that did shenanigans like that would stay in business?
Having a carburetor gummed up from gas sitting in it for several months is not beyond the realm of possibility. While replacement would be one way to fix that, I'd try running some cleaner (such as Seafoam) through it first to see if that gets it working well again. (Actually, I'd probably also take the carburetor apart and try cleaning it manually before buying a new one, too, but I wouldn't expect everybody to try that. I don't know how amenable these carbs are to this kind of work.)