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Sure but if the energy return is 15x the energy investment, carbon neutrality is pretty likely.

All energy is not created equal. In order to make this calculation in a useful way, one would need to know the amount of carbon emissions per kWh for both the panel production site and ones local grid. And it's actually even harder than that, as one also needs to know if one is lowering base-load generation needs or surge-capacity. Base- and surge-generation are frequently not the same as carbon emissions go.

Consider for instance that you live near the Hoover dam (desert, lots of sun, prime area for solar); your panels might never be carbon-neutral.

Sure, it's complex. Take your example. The Hoover dam sells power into California. So installing solar panels in Boulder City is not that different than installing them in Los Angeles (there would be some extra grid losses in LA). If the Hoover Dam had the capacity to supply the entire southwest it would matter, but it doesn't have that capacity.

There's also the question of whether buying Chinese solar panels makes China more or less likely to convert to solar itself.

Even in a 100%-coal grid, I don't think it's likely that >1000 kg CO₂/kWp is necessary. German manufacturers can achieve 400 kg CO₂/kWp in the still-rather-mediocre German grid.

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