1. No experience with the Linux client but I see another reply answering your question.

2. We have used cloud-based from the beginning and never had issues. Zulip's uptime is fantastic. There may have been one or two minor issues during the five year time period that we've used them.

3. We tried out the Jitsi integration when it was first introduced and it works fine but I don't think anyone at our company uses it since we have Google Apps for Business so everyone just uses Hangouts if we need video / voice.