Hahaha, no, we definitely don't expect to have the same parameters for mainnet. We are thinking of an expiry time of somewhere around one or two months for an Ark with real bitcoin.

Payments and other interactions within the Ark rely on the server. But all users have a unilateral exit package for each VTXO that they can broadcast if the server disappears or goes rogue.

See here: https://community.second.tech/t/330-sats-fee-for-sending-ln-and-boarding/98/2

The protocol plans around round times drastically changed. We (Second) are currently thinking about 1 hour rounds, more or less. But you would only use rounds to refresh your own VTXOs when they are about to expire. To send payments, you can your arkoor or out-of-round transactions that are instant. (But don't worry the above signet Ark server does rounds every 10 seconds.)

When it comes to doing coinjoin rounds, that's currently not implemented.

Nope, not yet. Not mobile at least. The OP is about a CLI wallet called bark, and it's on signet only for now. Mobile is our next milestone!