कधी जायचे?

It is neutral. You can use it with friends, family, and even colleagues.

Yes, the form 'jayche' doesn't change based on the number of people.

'Jayche' means 'to go', while 'nighayche' specifically means 'to leave/start'.

You can say 'Mala kadhi jayche?'

Yes, 'jayca' is the informal/colloquial version of 'jayche'.

You can answer with a time (e.g., 'Don vajta') or an event (e.g., 'Jevanavar').

Yes, in the context of time. In other contexts, it can mean 'ever'.

It's better to use 'आपण कधी प्रस्थान करणार आहोत?' for formal emails.

The '-ce' suffix indicates a gerundive of obligation or a planned future action.

Say 'Parat kadhi jayche?'.

Yes, very frequently, though often pronounced as 'Kadhi jaych?'.

Yes, that is the full, grammatically complete version.

There isn't a direct opposite, but 'ata' (now) or 'nantar' (later) are common answers.

Yes, many Marathi songs about travel or separation use this phrase.