What is the ruling on someone who swore not to do something and forgot to say "Insha'Allah" (God willing), then regretted it and wants to re-swear with the same wording but adding "Insha'Allah"; and is expiation for breaking an oath (kaffarat yamin) required for the first oath, or the second, or in both cases?

An exception made after an oath does not absolve one from expiation upon breaking the oath. Rather, for an exception to be validly connected to an oath, it must be continuous with the statement, without a long pause or extraneous speech. Once the ruling of an oath is established, it cannot be negated or altered by a subsequent exception.