Learn Hardcore Finnish: Bussi jäi ruuhkaan, joten saavuin kokoukseen myöhässä. - The bus got stuck in traffic, so I arrived late to the meeting.

Finnish often starts with what the sentence is “about” (topic), so Bussi is natural: The bus got stuck…
You could omit the subject if it’s clear from context, but here it would be unusual because Bussi jäi ruuhkaan is a complete, clear event statement. If you omitted it, you’d need something else (like context or a pronoun) to keep it natural.

What does the verb form jäi mean, and what is its dictionary form?

jäi is the past tense (imperfect) of jäädä = to remain / to get stuck / to be left.
So Bussi jäi ruuhkaan means the bus ended up stuck (or got caught) in traffic.

Why is it jäädä ruuhkaan? What case is ruuhkaan, and what does it add?

ruuhkaan is the illative case (roughly into). Illative often answers “where to / into what situation?”
With jäädä, Finnish commonly uses illative to express getting stuck into a situation:

jäädä ruuhkaan = get stuck in traffic
It’s not literally moving “into” traffic so much as becoming trapped by it.

Could it be Bussi jäi ruuhkassa instead? What’s the difference?

ruuhkassa (inessive, in) would describe being located in traffic, but it’s less idiomatic with jäädä for “got stuck.”
Typical patterns:

jäädä ruuhkaan = to get caught/stuck in traffic (idiomatic, event-focused)
olla ruuhkassa = to be in traffic (state/location-focused)

What is joten, and why is there a comma before it?

joten means so / therefore, introducing a consequence clause.
In Finnish, a comma is normally used before coordinating connectors like joten when it links two independent clauses:

Bussi jäi ruuhkaan, joten saavuin myöhässä.

Is saavuin a common way to say “I arrived”? What is its base form?

Yes. saavuin is the past tense of saapua = to arrive.
So saavuin = I arrived (completed event in the past).

Why is it kokoukseen and not kokous or kokouksessa?

kokoukseen is illative (“into/to the meeting”), used because saapua typically takes a destination:

saapua kokoukseen = arrive at the meeting (destination/event you arrive to)

By contrast:

kokous (nominative) wouldn’t fit after saapua in this meaning.
kokouksessa (inessive, “in the meeting”) would mean arriving while already inside the meeting context, and is less standard for the destination of arriving.

How do we get from kokous to kokoukseen? Why does it look like it has an extra kse?

This is a normal illative formation for many -s ending nouns.
kokous → stem kokoukse- → illative kokoukseen.
That -kse- element appears in several similar nouns when forming certain cases:

vastaus → vastaukseen
keskustelu behaves differently, but many -us/-ys nouns show this pattern.

Why is “late” expressed as myöhässä instead of myöhään?

They’re both possible, but they emphasize slightly different things:

myöhässä (inessive) = late as a state/condition: “I was late.”
myöhään (illative) = “to a late time,” often more like “(arrived) late” focusing on the time reached.

In this sentence, saavuin … myöhässä is a very common, natural way to say “I arrived late / I was late (arriving).”

Why is the order saavuin kokoukseen myöhässä and not saavuin myöhässä kokoukseen?

Both can be grammatical, but the default is often: 1) verb (saavuin)
2) destination/place (kokoukseen)
3) manner/state/time adverbial (myöhässä)

Putting myöhässä earlier can add emphasis to “late” or change the information flow. The given order sounds neutral and natural.

Does this sentence imply the speaker blames the traffic (the bus) for being late?

It strongly suggests causation: the bus getting stuck is presented as the reason (joten) for arriving late.
It doesn’t necessarily sound like “blame” in an emotional sense, but it does frame the lateness as a consequence of the traffic situation rather than, say, poor planning.