Αργή κίνηση

No, that would mean they are literally moving in slow motion like a movie effect. Use 'αργός' for a slow person.

It is neutral. It's used in news reports but also in casual conversation.

In context, they mean the same thing. 'Κίνηση' alone almost always implies 'heavy/slow traffic' when discussed as a problem.

You say 'Είμαι κολλημένος στην κίνηση.'

Technically 'αργές κινήσεις' exists, but it means 'slow movements' (like in dance or exercise), not 'multiple traffic jams.'

No, for internet we say 'αργή σύνδεση' (slow connection).

It's an impersonal construction similar to 'it's raining.' It implies the environment 'has' this condition.

In Athens, both are common, but 'μποτιλιάρισμα' is more specific to a complete gridlock.

It comes from the verb 'πήζω' (to curdle/solidify). It's a very vivid way to say the traffic is so thick you can't move.

Not really. For a slow pace of life, we say 'ήρεμη ζωή' or 'αργοί ρυθμοί'.

Usually no. It's specifically for road vehicles. For trains, we talk about 'καθυστερήσεις' (delays).

It's a military term for 'marking time' (marching in place). In traffic, it means moving very, very slowly.