I'm trying to stay out of this thread, but you're right at this part. Strictly speaking, if there were Tier One units available, they would've been under the command of either AFRICOM (or EUCOM, since AFRICOM has few tactical units at its disposal) or SOCOM through one of the Special Operations Joint Task Forces (JSOTFs).
But the part you didn't say here is that the US ambassador in a country has final say in all matters -- military and diplomatic -- unless countermanded by the President. So if (and I have no idea if this is the case or not) the Ambassador said "don't come here, it's too dangerous" or "I can hold them off with what i have" or "it's not that bad, don't bring troops here" then the DoD commanders can't send troops in without the President overriding the Ambassador. However, if the Ambassador's asking for help, then it's up to SECDEF to order his Combatant Commanders (AFRICOM, SOCOM, EUCOM, whoever) to accomplish the mission with whatever parameters (limit civilian casualties, bomb everything, take prisoners, whatever) he wishes.
I wish I could say more here, sorry.