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Spending US dollars outside of a city like Kathmandu or Pokhara will be difficult. And even within those cities, businesses accepting US$ will be limited in numbers (hotels, plus some restaurants and shops).
As pretty much all access to Nepal is via flights into Kathmandu, why not exchange US dollars for NP rupees upon arrival? One would think that a few minutes would be available to exchange money even on the tightest tour schedule (afterall the guide would want to make sure you have cash to tip the crew ;-)