Learn Hardcore German: Das Team arbeitet unter Zeitdruck, die Zeit ist knapp, doch mindestens zwei Kolleginnen helfen weiter. - The team works under time pressure; time is tight, yet at least two female colleagues keep helping.

Yes. In German you can separate independent main clauses with a comma when they are closely related (asyndetic coordination). Alternatives include a period, a semicolon, or adding a conjunction:

Das Team arbeitet unter Zeitdruck. Die Zeit ist knapp.
Das Team arbeitet unter Zeitdruck; die Zeit ist knapp.
Das Team arbeitet unter Zeitdruck, und die Zeit ist knapp. The comma before doch later in the sentence is also required, because doch is a coordinating conjunction linking two main clauses.

What does doch mean here, and how does it differ from aber or jedoch?

doch here means "but/however/yet" and is a coordinating conjunction. It sounds a touch more contrastive or concessive than aber, and often softens bad news with a positive twist.
aber is the most neutral "but" and is fully interchangeable here: ..., aber mindestens zwei Kolleginnen helfen weiter.
jedoch is more formal and is a conjunctive adverb, not a conjunction. It often stands in the first position of the clause, so the verb follows it: ..., jedoch helfen mindestens zwei Kolleginnen weiter. You can also place it mid-clause: ..., mindestens zwei Kolleginnen helfen jedoch weiter.

Which case is used in unter Zeitdruck, and why is there no article?

unter is a two-way preposition; with static situations it takes the dative. The underlying form would be dative singular: unter (dem) Zeitdruck.
In the fixed expression unter Zeitdruck (arbeiten/stehen) the article is normally omitted because the noun is abstract/generic. You would only say unter dem Zeitdruck if you refer to a very specific, previously mentioned pressure.
Note: Zeitdruck is masculine (der Zeitdruck).

Why knapp in die Zeit ist knapp and not kurz?

knapp means "scarce/tight/insufficient." With time or money it means there isn't enough of it.
kurz means "short" in length or duration (e.g., a short meeting), not "insufficient." So for time pressure you say Die Zeit ist knapp, Es ist knapp mit der Zeit, or Wir haben wenig Zeit, not Die Zeit ist kurz.

Can I omit the article and say Zeit ist knapp?

Yes. Zeit ist knapp is grammatical and idiomatic as a general statement. Die Zeit ist knapp points to the specific time available for this task/situation. In many contexts both are fine; the version with die is slightly more situationally anchored.

Why is it Das Team (neuter) and singular? Shouldn't a team be plural or have a gender?

The loanword Team is neuter in German, so it takes das and is singular: Das Team arbeitet ...
When you refer back to it, use es (not sie/er): Das Team hat viel zu tun. Es arbeitet unter Zeitdruck.
If you want to talk about the people, switch to a plural noun: die Teammitglieder arbeiten ...

What exactly does weiterhelfen mean compared to plain helfen?

helfen = "to help."
weiterhelfen = "to help (someone) further / to be of help"—often used when you can't solve everything but can move things along, give useful information, or support the next step. Examples:
Kannst du mir helfen? "Can you help me?"
Das hat mir sehr weitergeholfen. "That really helped me (move forward)."

Why is it written helfen weiter here instead of weiterhelfen?

Because weiterhelfen is a separable verb. In main clauses, the finite verb is in 2nd position and the separable particle goes to the end:

Main clause: Mindestens zwei Kolleginnen helfen weiter.
Subordinate clause: ..., dass mindestens zwei Kolleginnen weiterhelfen.
Perfect tense: Die Kolleginnen haben weitergeholfen. In the dictionary form (infinitive), it's written together: weiterhelfen (not weiter helfen).

Does weiterhelfen take a dative object? Where is it in this sentence?

Yes. Like helfen, it takes a dative object: jemandem (weiter)helfen.

With an explicit object: Mindestens zwei Kolleginnen helfen dem Team weiter.
In the given sentence, the object is omitted because it's clear from context (they are helping the team).

Why Kolleginnen (female colleagues)? How can I make it gender‑neutral?

Kolleginnen is the feminine plural, explicitly "female colleagues." Common neutral or inclusive options:
Generic masculine (traditional, contested): mindestens zwei Kollegen helfen weiter (could mean male or mixed).
Pairing: mindestens zwei Kolleginnen und Kollegen helfen weiter.
Gender‑inclusive punctuation: mindestens zwei Kolleg:innen (or Kolleg*innen), depending on house style.
Avoid gendered nouns: mindestens zwei Teammitglieder helfen weiter. Singular forms: die Kollegin (f.), der Kollege (m.; plural: die Kollegen).

What is the difference between mindestens, wenigstens, and zumindest?

mindestens = at least (numeric lower bound). Use with numbers/quantities: mindestens zwei Kolleginnen.
wenigstens = at least (consolation/minimum acceptable outcome): Es klappt nicht, aber wenigstens versuchen wir es.
zumindest = at least/at any rate (concessive limitation): Wir sind zu spät; zumindest helfen zwei Kolleginnen weiter. In your sentence, mindestens is the right choice because it's about a number.

How could I express the same contrast with a subordinate clause like obwohl?

Use obwohl ("although"), which sends the finite verb to the end of its clause:

Obwohl die Zeit knapp ist, helfen mindestens zwei Kolleginnen weiter. You could also make the time‑pressure part subordinate:
Das Team arbeitet unter Zeitdruck, obwohl mindestens zwei Kolleginnen weiterhelfen.

Is unter Zeitdruck the only idiomatic way to say this? What about im Zeitdruck or other phrases?

The idiomatic expression is unter Zeitdruck (arbeiten/stehen/sein). Im Zeitdruck is not idiomatic.
Other natural options:
Zeitdruck haben: Das Team hat Zeitdruck.
unter Druck stehen/sein (more general): Das Team steht unter Druck.
Informal: im Stress sein: Das Team ist im Stress.