Your expression "prently" seems perfectly understandable as a shortened "presently." However, I suppose it is in danger of acquiring a new meaning. At least this is what has happened to "ornry" or "onery" (obstinate) for "ordinary" and "ignernt" (rude) for "ignorant" (of manners) and probably a few others.

Paul's thinking on "a whole nother" is to give it the label of "infixation." Normally such a label would be confined to languages that use it as a regular device like we do "prefixation" and "suffixation." Herb's explanation of "re-analysis" has the virtue of yielding other examples that may at least start to make it a device found elsewhere in English. I do not think "a whole nother" means the same as "an other whole" rather to me it is interpreted as "a wholly other." The use of "whole" as an adverb is not wholly unheard of, so my feeling is that the use of the "n" on "other" is 1) to mark "whole" as an adverb, 2) to preserve "other" as a normally non-gradable adjective (we don't say, *"a very other") and 3) to include with "nother" its own sense of "other variety." I believe that we are witnessing here again the force of change expressed by the dictum "different form, therefore different meaning." First there is a perception of a different style or register, but later it solidifies as a new vocabulary item with its own meaning.

I can't help but mention a habit in the vernacular where I have recently relocated. They regularly use "aggravate" for "exaspirate." This has always irritated me in the past, but probably because of my parents or English teachers. It is quite widespread and as Fowler points out it was in use as early as 1611. The slight difference in meaning is useful and easily gathered from context. Alas, such are the normal winds of change.