Learn Hardcore Greek: Αύριο ο συνάδελφός μου θα δουλεύει από το σπίτι όλη μέρα. - Tomorrow my colleague male will be working from home all day.

Because the weak possessive μου is an enclitic. When an enclitic follows a word stressed on the antepenultimate syllable (like συνάδελφος), Greek adds a second accent on the final syllable to keep the stress within the last three syllables. Hence: ο συνάδελφός μου.
Compare:

ο άνθρωπός μου (antepenultimate → adds a second accent)
ο φίλος μου (penultimate → no extra accent)

Why is the possessive μου placed after the noun? Can I say ο μου συνάδελφος?

In Greek, the weak possessive pronouns (μου, σου, του/της, μας, σας, τους) follow the noun phrase: ο συνάδελφός μου. You cannot put μου in front. If you need emphasis or a “my own” meaning, use the strong form δικός:

ο δικός μου συνάδελφος = my colleague (as opposed to someone else’s)

Do I need the article ο even though I already say “my colleague”?

Yes—Greek typically keeps the definite article with a noun that has a possessive: ο συνάδελφός μου. Omitting it (συνάδελφός μου) is possible in certain styles (e.g., headlines, vocatives, after “είμαι”), but the normal, neutral form includes the article.

What nuance does θα δουλεύει have compared to θα δουλέψει?

Greek future distinguishes aspect:

θα δουλεύει (imperfective): “will be working,” ongoing or throughout a period; fits naturally with όλη μέρα.
θα δουλέψει (perfective): “will work (once, to completion).” With όλη μέρα it sounds odd unless you mean “he’ll manage to work for the entire day (as an accomplishment).”

Could θα δουλεύει also mean “he is probably working”?

Yes. Without a time marker, θα δουλεύει often expresses conjecture about the present: “he must be working.” In your sentence, Αύριο forces the future-time reading: “Tomorrow … will be working.”

Why is it από το σπίτι and not στο σπίτι or just σπίτι?

από το σπίτι = “from home,” the standard expression for remote work (work-from-home).
στο σπίτι = “at home,” simply locating the activity at the house; it can be fine contextually, but for WFH the idiomatic choice is από.
Bare σπίτι (without article) works adverbially in some verbs like είμαι σπίτι (“I’m at home”), but with από you need the article: από το σπίτι.

Can I shorten από το σπίτι?

Yes, in speech and informal writing απ’ το σπίτι is very common (elision of the vowel in από before a consonant). You still keep the article. Avoid από σπίτι here.

Why is it όλη μέρα? Is όλη τη(ν) μέρα also correct?

Both are common:

όλη μέρα is an idiomatic, article-less time expression (“all day (long)”).
όλη τη(ν) μέρα is also correct and a bit more explicit.
About τη(ν): the final -ν is optional before many consonants; with μέρα, you’ll see both τη μέρα and την μέρα. A safe, very common choice is όλη τη μέρα.

Is there any difference between μέρα and ημέρα?

They mean the same; ημέρα is more formal. So you can also say όλη την ημέρα for “all day (long).”

Can I change the word order?

Yes, Greek is flexible with adverb placement:

Αύριο ο συνάδελφός μου θα δουλεύει από το σπίτι όλη μέρα. (given)
Ο συνάδελφός μου αύριο θα δουλεύει από το σπίτι όλη μέρα.
Αύριο όλη μέρα ο συνάδελφός μου θα δουλεύει από το σπίτι. (emphasis on duration) All are acceptable; fronting Αύριο is very natural.

Can I use the present for a scheduled future, e.g., Αύριο … δουλεύει?

Yes. Greek often uses the present for near-future plans/timetables:

Αύριο ο συνάδελφός μου δουλεύει από το σπίτι όλη μέρα.
This sounds like a scheduled arrangement. Using θα δουλεύει highlights the future viewpoint and the ongoing nature.

Is συνάδελφος only masculine? How do I refer to a female colleague?

συνάδελφος is common gender. For a woman you typically use the feminine article:

η συνάδελφός μου (note the extra accent because of μου)
Plural for all genders: οι συνάδελφοι.
You may also see συναδέλφισσα (feminine form), but η συνάδελφος is very common today.

Any pronunciation tips for tricky parts?

Αύριο: “AV-rio” (αυ before a voiced consonant sounds like “av”).
δουλεύει: “thu-LE-vi” (δ = th in “this”; ευ before a vowel/consonant like λ often sounds “ev”; ει = “i”).
απ’ το: the final vowel of από drops and the p+t meet: “ap-to.”
A natural overall rendering: “AV-rio o si-NÁ-del-fos mu tha du-LÉ-vi apó to SPÍ-ti Ó-li MÉ-ra.”

Is a comma needed after Αύριο?

Not required. A short fronted time adverb like Αύριο usually takes no comma unless you want a rhetorical pause or strong emphasis.

Could I say θα εργάζεται instead of θα δουλεύει?

Yes. εργάζομαι is a more formal/neutral verb for “work.”

Αύριο … θα εργάζεται από το σπίτι όλη μέρα.
Both are correct; δουλεύω is more colloquial in everyday speech.