Answers:
In this sentence: Jemand Unbekanntes, Unbekanntes is an adjectival noun. What is the role of jemand? Is it an adjective? A pronoun?
Any grammar will tell you that jemand (and niemand) are indefinite pronouns. So, no adjective. Definitely not. (BTW: I don't like your term of an adjectival noun - Rather call it a substantivized adjective, because that is what it is.)
Can we use jemand as an adjective? like "jemand Mann"?
Also definitely not. You cannot use pronouns as adjectives.
If jemand comes after the determiner, how is it declined? Example: "solch ein Jemand ist unserer Beobachtung wert". Is it declined like an adjective?
Solch ein jemand is a grammatical oxymoron and just about as wrong as "such an anyone". You cannot address a specific anyone. This would be possible with the Substantive "Niemand" (yes, the other indefinite pronoun has a substantivated counterpart, also like in English) such as in
Komm' mir bloß nicht auf die Idee, so einen Niemand zu heiraten.
Meaning a person of no relevance. This does not apply to jemand, though.
And yes, in other situations, the indefinite pronouns niemand/jemand are declined, just like any other pronoun (but not like adjectives):
(N)Jemand muss das Geschirr abwaschen.
(G)Er wollte nicht jemandes Diener sein.
(D)Die Hausaufgaben habe ich bei jemandem in der Schule abgeschrieben.
(A)Ich muss heute abend noch dringend jemand(en) treffen.
But they are normally not declined if they come with a substantivized adjective that carries the declension (so it's either the pronoun that is declined or the substantivated adjective, but normally not both)
(G)Er wollte nicht jemand Unbekanntes Diener sein / jemandes Unbekannten (This is pretty uncommon)
(D)Die Hausaufgaben habe ich bei jemand Fleißigem abgeschrieben./ bei jemandem Fleißigen
(A)Ich muss jemand Reichen treffen. / jemanden Reichen