A Few Tau quesitions

Hi there, after playing a few games against Tau a have a few questions, I was hoping I could get some answers to.

1. Markerlights. Can someone explain to me how they are supposed to work? Can you really put one markerlight on and have the whole army use the benefit of that one markerlight?

2. Can the commander and retinue purchase an item that lets them not scatter when they deepstrike?

3. Can they commander have two blast template (one being large) weapons, plasma and melta guns, along with the onager(not sure if that's its name) gauntlet?

4. I thoroughly dislike the amount of ignoring cover, line of sight, ap4 Twinlinked missile they put out...

I would check myself, though I don't have access to the codex myself, but geez I see why people don't like playing Tau.