Is a Muslim permitted to swear a negative oath falsely to avert harm that would result from telling the truth?

You have done well by repenting and feeling remorse. Lying to avoid harm is permissible if Tawriyah (equivocation) and Ma'areedh (indirect speech) are not possible. Otherwise, one must resort to them instead of lying, for "in Ma'areedh there is enough to spare a Muslim from lying."