Learn Hardcore Indonesian: Dia datang lebih awal; bahkan menyiapkan kursi untuk tamu. - He/She arrives early; he/she even prepares chairs for the guests.

Bahkan means “even,” adding emphasis that the second action is an extra, somewhat surprising step on top of the first. It highlights escalation: not only did the person arrive early, they also prepared chairs.

Neutral–formal: bahkan
Informal: malah
Slightly more formal/literary: malahan

All three would work similarly here:

Dia datang lebih awal; bahkan menyiapkan kursi untuk tamu.
Dia datang lebih awal; malah menyiapkan kursi untuk tamu.
Dia datang lebih awal; malahan menyiapkan kursi untuk tamu.

Is the semicolon appropriate, or should I use a comma or period?

A semicolon is acceptable but a bit formal. Strictly speaking, semicolons usually join two independent clauses; the second part here omits the subject (dia), so many editors prefer a comma or a period.

Very common: Dia datang lebih awal, bahkan menyiapkan kursi untuk tamu.
Or split: Dia datang lebih awal. Bahkan, dia menyiapkan kursi untuk tamu.
If you keep the semicolon, adding the subject and a comma after bahkan feels polished: Dia datang lebih awal; bahkan, dia menyiapkan kursi untuk tamu.

Why is the subject missing in the second clause? Should I repeat dia?

Dropping the repeated subject is natural in Indonesian when it’s clear from context. Repeating dia is optional and can add clarity or emphasis.

Ellipsis (natural): …; bahkan menyiapkan kursi untuk tamu.
With repetition (clear/emphatic): …; bahkan dia menyiapkan kursi untuk tamu.

Does lebih awal mean “earlier” or “early”? Are there other options?

Lebih awal means “earlier” (relative to an expected time or to others). For nuances:

Earlier than others/expected: lebih awal
Early in the morning: pagi-pagi or pagi hari
Earlier/first (sequence): lebih dulu/dahulu or more formal terlebih dahulu

Examples:

Dia datang lebih awal.
Dia datang pagi-pagi.
Dia datang terlebih dahulu.

Can I say lebih cepat instead of lebih awal?

Not usually. Cepat refers to speed; awal refers to the point in time.

Arrived earlier (time): Dia tiba lebih awal.
Arrived faster (the trip took less time): Dia tiba lebih cepat.

Do I need a past marker like sudah to show this happened in the past?

Indonesian has no tense inflections; time is inferred from context. You can add aspect/time markers if needed:

Completion: sudah (neutral), telah (formal)
Recent past: tadi, barusan Examples:
Dia sudah datang lebih awal; bahkan menyiapkan kursi…
Tadi dia datang lebih awal…

Is menyiapkan the best verb here? How does it differ from menata, menyusun, or menyediakan?

Menyiapkan (from siap) = to prepare/set up something (broad, good default).
Menata/menyusun = to arrange/lay out neatly (focus on arrangement).
Menyediakan = to provide/supply (focus on availability, not the act of arranging).
Mempersiapkan = more formal synonym of menyiapkan.

So:

Setting chairs in place: menyiapkan kursi or menata kursi
Ensuring chairs are available: menyediakan kursi

Is untuk the best preposition? How about bagi or buat?

All can mean “for,” but with different feels:

Untuk: neutral, widely used for purpose/recipient. Good here.
Bagi: more formal/abstract (policies, principles), but fine with people too.
Buat: informal colloquial.

So:

…kursi untuk tamu. (recommended)
…kursi bagi tamu. (a bit formal)
…kursi buat tamu. (casual speech)

Is tamu singular or plural here? Should I say para tamu or tamu-tamu?

Nouns are unmarked for number, so tamu can be singular or plural from context. To make plural explicit:

Human plural (collective): para tamu (common and natural)
Reduplication: tamu-tamu (grammatical but less common in formal prose)

Example: …kursi untuk para tamu.

Can I use ia instead of dia? What about beliau?

Dia: neutral, everyday, subject or object.
Ia: more formal/literary, typically used as a subject (rare as an object).
Beliau: honorific for respected figures/elders; not for peers or yourself.

All subject-wise are possible here depending on register:

Ia datang lebih awal; bahkan menyiapkan… (more literary)
Beliau datang lebih awal… (only if showing respect)

Can I move bahkan to other positions?

Yes; bahkan typically precedes the element it emphasizes.

Emphasize the added action: Dia datang lebih awal; bahkan menyiapkan kursi…
Emphasize the subject as surprising: Dia bahkan datang lebih awal, menyiapkan kursi…
With repetition for clarity: Dia datang lebih awal; dia bahkan menyiapkan kursi… Avoid overusing conjunctions like dan with bahkan unless needed for rhythm: …dan bahkan… is possible but often unnecessary.

Is it okay that datang has no destination? Should I add ke?

Yes, datang can stand alone when the event/location is understood. Add a destination when needed:

Dia datang lebih awal ke rapat.
Dia datang lebih awal ke rumah saya.

How would a passive or fronted-object version look?

Several natural options:

Passive with -nya clitic: Bahkan, kursi untuk tamu disiapkannya.
Passive with oleh: Bahkan, kursi untuk tamu disiapkan olehnya. (more formal)
Object fronting (active): Bahkan, kursi untuk tamu ia siapkan. (emphasis on the chairs)