A colega com quem partilho casa disse que, se acabarmos hoje, jantamos cedo.

Yes. Colega can refer to either a male or a female colleague, but the article shows the gender:

• a colega = a female colleague / the female colleague
• o colega = a male colleague / the male colleague

So the noun itself does not change here, but the article does.

Because a colega simply means the colleague. Portuguese has not explicitly said my colleague here.

In context, English might naturally translate it as my colleague if it is obvious whose colleague she is, but the Portuguese sentence itself only says the colleague. If the speaker wanted to be explicit, they could say a minha colega.

Because the idea is partilhar casa com alguém — to share a home with someone. When a relative pronoun refers to a person after a preposition, Portuguese normally uses quem:

• a colega com quem partilho casa

Literally, it is close to the colleague with whom I share a home.

Com a qual is grammatical, but it sounds more formal and heavier:

• a colega com a qual partilho casa

Com quem is much more natural here.

As for a colega que partilho casa com, that is not the standard structure learners should aim for. In Portuguese, the preposition normally stays before the relative pronoun:

• com quem
• not usually que ... com like in informal English the person I live with

It means to share a home / to live in the same home as someone. In natural English, this is often to share a flat/house with someone or to be housemates.

It does not necessarily mean shared ownership. It just means they live in the same place.

Partilhar casa is a very common expression meaning to share a home in a general sense.

Without the article, it sounds more idiomatic and focuses on the living arrangement.
With the article — partilho a casa — you are more likely to be thinking about a specific house as an object.

So:

• partilhar casa = share a home / live together
• partilhar a casa = share the specific house

In Portuguese, casa is often used in a broad sense meaning home or the place where someone lives, not only a detached house.

So partilhar casa can still be used even if the people actually live in a flat/apartment.

Because disse que is the normal way to introduce reported speech in Portuguese:

• Ela disse que... = She said that...

Portuguese usually keeps que here. English often drops that, but Portuguese normally does not.

So disse que is the standard structure.

Yes. Here acabarmos is the future subjunctive.

After se referring to a possible future situation, Portuguese normally uses the future subjunctive:

• se acabarmos hoje = if we finish today

This is a very important pattern in Portuguese:

• se eu for
• se tu tiveres
• se nós acabarmos

For regular verbs, some future subjunctive forms look identical to the personal infinitive, but here the function is clearly future subjunctive because it follows se and expresses a future possibility.

Because Portuguese very often uses the present indicative to talk about the near future, especially in everyday language.

So:

• jantamos cedo = we eat / we’re eating / we’ll eat early
• jantaremos cedo = also correct, but more formal, more explicit, or less conversational

In a sentence like this, jantamos cedo sounds very natural.

Portuguese does not always backshift tenses in the same way English does.

Here, the colleague spoke in the past, but the finishing and the dinner are still future possibilities relative to the situation being discussed. So Portuguese can keep:

• disse que, se acabarmos hoje, jantamos cedo

That sounds natural because the condition and result are still seen as relevant and still in the future.

Because se acabarmos hoje is inserted into the middle of the reported clause.

The basic reported idea is:

• disse que jantamos cedo

The conditional clause interrupts that:

• disse que, se acabarmos hoje, jantamos cedo

So the commas help mark that interruption.

Compare:

• Se acabarmos hoje, jantamos cedo.
• Jantamos cedo se acabarmos hoje.

When the se clause is in the middle, the commas are especially helpful.

Because Portuguese often leaves subject pronouns out when the verb ending already shows who the subject is.

Here:

• partilho = I share
• acabarmos = we finish in this se clause
• jantamos = we eat / we’ll eat

So Portuguese does not need to say eu and nós unless there is some special emphasis or contrast. This is very normal.