Generally, when past participle conjugations are used, the participle is used with a conjugation of "haber" as the sentence's verb. Example: "Te has dedicado como maestro." "Haber" conjugation is "has", participle is "dedicado." In the sentence given there's no "haber" conjugation to indicate a participle conjugation, therefore it's modifying the noun (adjective). Hopefully it didn't get too messy!
Another difference is that when the past participle is used with haber it remains unchanged while the conjugation of haber determines who/what has/had done the action. However, when it is used as an adjective it must match gender and number with the noun it is modifying. Eg: They have cooked the onions = Han cocinado las cebollas. They eat the cooked onions = Comen las cebollas cocinadas.
You propose three distinct uses, but in reality there are only two. "Dedicado," depending on the context can act (1) as a past participle verb or (2) as an adjective. It's not both simultaneously nor that you use one as the other.
The present perfect tense is a compound tense; it requires a past participle conjugation and a simple present conjugation of "haber." So this is the same as the (1) above.
I hope it's a bit clearer now.
Me too. You can only hear it if you play the slomo, otherwise it sounds like "el es"
It's tricky because "Serious" in English has several different meanings that can translate into different words in Spanish (there is some overlap in Spanish as well but the following examples are just intended to highlight a point).
My grandfather is serious - Mi abuelo es serio
She is a serious actor - Ella es una actriz dedicada
The situation is serious - La situación es grave
It is a serious wound - Es una herida importante
Anyway, the point is that "serious" can be used for "dedicado" when it means "dedicated." So you have to ask could "You are a serious teacher" mean "You are a dedicated teacher." I guess it's possible. On the other hand, could "You are a serious child" mean "You are a dedicated child." Not so much. So if what you're saying is correct then I think DL has it around the wrong way.
I was taught that maestro means conductor ... like of an orchestra? Can it mean that also?
Wait isn't the verb 'ser' used to describe a permanent quality or status you have very little power to change? If a teacher was dedicated, a. She can't be born a teacher b. She can always decide to stop. Wouldn't ella está un maestro dedicado be more gramatically correct