This is chapter 2 in the story of APA (Avoid Potential Ambiguity). Here I'm going to look at whether the advice is useful. Suppose you convince J. Doe that some usage is to be avoided because it "could lead to ambiguity". Will Doe's speech and writing now be clearer?
Well, no. (They could even be a little bit harder to understand.)
Let's take a specific case, sentence-adverbial (speaker-oriented rather than subject-oriented) hopefully, as in
(1) Hopefully, you didn't open the box.
The first thing to say about (1) is that these days no rational person who speaks English fluently could misunderstand the probable import of hopefully in (1): 'I hope (that) you didn't open that box', 'It is to be hoped that you didn't open that box', etc. And not 'You were hopeful in not opening the box'. (You CAN construct contexts in which this understanding is possible, but they require working against default assumptions about the way the world works, about people's intentions, and the like.)
To understand the probable import of hopefully in (1), you have to know that lots of people use hopefully as a sentence adverbial; it's all over the place. (I haven't run the numbers, but I'm sure that these days sentence-adverbial hopefully vastly dwarfs nominal-modifying hopefully in both colloquial and more elevated English.) So EVEN IF YOU DON'T USE IT THAT WAY YOURSELF, you have to be prepared to understand what other people are doing with it. Behaving otherwise is the equivalent of sticking your fingers in your ears and shouting "I can't hear you! I can't hear you!" It's childish and uncooperative.
And there's plenty of evidence that people who disparage or proscribe certain usages do in fact understand clearly what others who use them intend by them. As Geoff Pullum pointed out to me some time ago in connection with the proscription of logical since and while, copyeditors appear to be virtually perfect in detecting which instances of since and while are to be replaced. But how could they do this if they didn't understand the writers' intentions? If the originals were in fact "ambiguous", how could the editors sniff out these intentions so reliably?
[Many times the objections to logical since/while (or sentence-adverbial hopefully or many dozens of others) are framed: this usage is just NOT ENGLISH. As a statement of fact, this is absurd. But it's not a statement of fact, it's a normative judgment: ENGLISH SHOULD NOT BE THIS WAY. Why not? Because of APA (often backing up some other prejudice against the usage, as innovative, lower-class, or whatever). This is where we came in.]
Ok, back to J. Doe. Assume that you and Doe have taken to heart one of these APA-based proscriptions. Suppose the two of you avoid sentence-adverbial hopefully scrupulously. What do other people make of this?
They don't get it. People hardly ever notice that others do NOT use some variant, so long as they understand the variants people do use. And of course they interpret things within their own grammatical system, so that they contemplate two uses for sentence-initial supplementary hopefully, no matter what you intend.
Which means that some of your readers/hearers might pause for a few milliseconds over your sentence-initial hopefully, expecting it to be a sentence adverbial and then deciding it was a nominal modifier (the rarer variant).
[I am NOT saying that if it happens — this is to be demonstrated — this pause is a bad thing. It just is. Sentence processing involves holding off options, juggling alternatives, revising expectations, and much else. Utterly uncontroversial variants often differ slightly in processing time. So what? Why should we be concerned over millisecond, or even centisecond, differences?]
If you really wanted to ensure that your English would be clearer because you used hopefully (since/while, etc.) in only one way, you'd have to ensure that almost everyone else did the same; otherwise, they'll interpret what you say in terms of their own linguistic system — and you should know that.
In fact, the only way you can ensure that other people understand your intentions is either to avoid nominal-modifying hopefully (as well as sentence-adverbial hopefully) and spell out the intended meaning, replacing (2) below by something like (2′),
(2) Hopefully, Kim opened the box.
(2′) Kim was hopeful in opening the box.
or else to gloss the meaning with extra verbiage:
(2″) Hopefully — that is, with hope — Kim opened the box.
(There are not a great many occurrences of "original" decimate in the last century or so, but many of the ones you can collect have explicit glosses pointing to the 'reduce by one-tenth' interpretation — because, of course, most people will understand decimate in the long-dominant 'reduce greatly, devastate' sense.)
Note that in many cases, clarity will then be bought at the cost of brevity; one of the several virtues of sentence-adverbial hopefully is its brevity, after all.
But just using hopefully only as a nominal modifier won't make your writing and speech any clearer.