The reason he lost credibility is that he seemed to be ignoring the constitution, that passed the power to the VP etc. The press hated him so destroyed him as being power hungry, but he was actually trying to assure the country to stay calm.
However, if the shooting of Reagan had been a communist plan, i.e. prior to an attack on the USA, Haig would have been right.
as for Tillerson, one reason to agree with Haley is that Trump fired him when he was overseas, right after he assured the Euroweenies that he would stop Trump from changing things in NATO etc.
Haig was in fact in Operational Control of all forces until Bush returned. You notice the Speaker of the House’s name was never mentioned — that’s because this was not yet a Succession question.