The original form of اما بعد is as below:
مهما يكن من شيء بعد الحمد و الصلاة
In rough translation: “Whatever may be, after praise and prayers…”
مهما is mubtada so it must be an ism.
يكن is a condition/shart, hence in jazm. A ف is required after a condition in most cases.
اما is a harf that is replacing the mubtada (مهما) and the condition/shart (يكن). So, اما is the laazim, and مهما and يكن are malzoom.
A ف is required after اما and also the attachment of an ism so that the effects of the مهما and the يكن are maintained.
بعد is an adverb of time. In اما بعد although بعد has the sign of raf’ it is actually in a state of Nasb due to being governed by يكن. Adverbial structures usually occur in a spoken-idafah form e.g. بعد الصلاة. But when the adverb appears alone and the mudaf ilayh is omitted, it becomes mabni. And when it is mabni, it is given the sign of raf’ although its actual grammatical state may be different.