1. No, not that I know of. They have to be twitch chat to earn currency. As long as they linked their discord/twitch together, they will be able to use commands, even if they require currency.
2. Yes they could. Basically when they link their twitch account to discord, deepbot will sync whatever user group they are to discord. Example: If they are VIP gold, it will make them VIP gold on discord.
3. Yes, they can request songs, as long as the bot is listening to that channel. It works the same way with twitch chat. So to answer your question on amount and song skips, yes it does.
4. They should be able to, but it may or may not respond back that the command has been made in discord. It will respond in twitch chat if anything.
5. Yes, they do. They just may not respond back in discord, unless that was fixed/updated.
It functions the exact same way it would in twitch chat. Just know some of the built in commands may not respond back in discord. As far as I know, nothing is really missing except live twitch chat in discord (which isn't possible with discord anyways). Feel free to ask any more questions.