"Verbs are by nature predicating words that require one or more nouns or noun phrases to complete their meaning." The linguist treatment is to classify Chinese adjectives as verbs. Adjectives differ with (in)transitive verbs in some syntactical behaviors in the sense that they sometimes behave like nouns.
According to Pulleyblank, a test by 可 and 可以 can help us figure out the part of speech.
Only transitive verbs can follow 可 (possible) and must be understood as passive. For example, 人可杀 = the man may be killed.
A transitive verb in an active sense, or an intransitive verb requires the compound word 可以 (the passive role is actually fulfilled by 以, and the meaning of instrument is extended to include agency). For example, vt. 王可以杀人 = The king can kill a man. vi. 你可以来 = You can come.
Like nouns, adjectives require the copula 为 after 可以. For example, 可以为美乎？
This test might not be perfect but it does offer a first way to try.
清楚 does have transitive usage, e.g. 我很清楚你的想法=I'm very clear about what you think, but it doesn't pass Test 1. 电视可以清楚 is grammatical and 电视可清楚 is not (电视可清楚(吗)？ and 电视可清楚了 are grammatical, but another two different usages of 可 are used). From this, 清楚 could primarily be an intransitive verb.
Similarly 高兴 is an intransitive verb since 你可以高兴 is grammatical and 你可高兴 is not (again, 你可高兴？ and 你可高兴了 are grammatical, but another two different usages of 可 are used).
The notion of "an adjective which is converted from a verb" is misleading in Chinese. Adjectives, transitive verbs and intransitive verbs belong to the same category. In particular, adj.'s and vi.'s fulfill very similar syntactical roles, and in cases that encounter regularly no conversion is done behind the curtain.
Adjectives are sometimes called "stative verbs" but there are objections, since transitive verbs such as 知 also denote a state rather than an action.
兼属 is normally called 兼类, that is a word has multiple parts of speech. It can be both adj. and vt. like 高兴, or n. and vt. like 上, and etc.
Be careful not to confuse it with 兼词, which is which is one character representing the combination of two words, e.g. 甭 béng = 不用，叵 = 不可.
The borderline between Chinese parts of speech is vague compared to that of English. Many words are 兼类, and even if they are not, case by case they might be used temporarily as another part of speech. Especially between vi.'s and adj.'s, it's usually, in my opinion, not necessary to distinguish.
I've never thought about the use of 地. Probably you can give more examples so that we can see whether my following intuition is correct. Simply from 清楚 or 高兴 versus 快，多, for me the distinction lies in disyllabic (with or without 地, case-depending) vs. monosyllabic (without 地).
UPDATE
Linguistically, Chinese adjectives are treated as a subclass of verbs, while intransitive and transitive verbs being another two subclasses. This is mainly based on syntax, and the most obvious reason is that adjectives can predicate on their own.
In many usages, it's even not possible for native speakers with no linguistic training to distinguish vi.'s and adj.'s. For example, 我高兴, native intuition doesn't help.
In 他可以漂亮，他可以帅, 漂亮 and 帅 are used temporarily as vi.'s. On the one hand, 可以 cannot be followed by adjectives. On the other, I would subconsciously understand them as (显得/看起来)漂亮/帅, which is a temporary vi. use (this might require a certain level of 语感). I agree Test 3 is unnatural for modern language. Luckily, we can usually distinguish vi's and adj.s by Test 2, similar to my thought process for 漂亮/帅.
Dictionaries list adjectives as adjectives, (in)transitive verbs as verbs. This is consistent with dictionaries of other languages. 《现代汉语词典》 provides parts of speech. If you look at the examples for the 动 entry of 清楚 and 高兴, they all include transitive usages. This makes listing a 动 entry necessary because adj. cannot naively be followed with an object.
我真不清楚他为什么要这样做。他就是高兴看电影。
As for 厉害, it's not transitive. By Test 2, 他可以厉害 is unnatural naively but 厉害 could also be interpreted as being temporarily used as vi.'s. It then falls into adjectives. Note that temporary changes of part of speech are restricted in modern speech and often considered wrong. Their usages should be avoided unless you're proficient in the language. By the few examples I've looked at, 《现代汉语词典》 does make a distinction between adj. and vi., in that adj. is noted by 形 and vi. and vt. by 动.
