Are you saying that the adverbial phrase in this case is supposed to be はっきりとした,not just はっきりと? Am I correct to say that the former is strictly prenominal adjective whereas the latter is purely an adverb?
I didn't say adverbial phrase, I said verbal phrase, a.k.a. a short embedded sentence. Which does mean the entire phrase 「はっきりとした」.
But your conclusion is correct; phrases like this can be used as adjectives, while the はっきり・はっきりと are adverbs.
I've not seen any definitive source on the meaning of using と with adverbs that can optionally take a と, but I would say from my experience that the difference does not seem to be very large. If pressed, I would say I think that using Xと is indicating the thing is best described, or most fittingly described by the adverb X, while using the same adverb X without と simply indicates without any particular emphasis that this is
an attribute of the thing described.
Could any adverb that take とparticle be structured the same way as はっきり,for instance さっさ?
I'm not sure what you mean by this. Any adverb can be used to modify verbs... that's the definition of an adverb.
In the case of とした, however さっさとする means 'to do quickly' or 'to do promptly'. I believe the phrase is always treated as an action verb, and never as a stative verb, so さっさとした cannot be used descriptively to mean "quick" or "prompt" in the same way that はっきりとした can be used to mean "clear".
You can of course grammatically use さっさと to modify a verb that then modifies a noun, as long as the meanings make sense for the words involved like,
さっさと終わらせた仕事 for "work that was finished quickly".