Can my Guardian also have a Longbow as a weapon?
they cant but if you want range try scepter with torch, it can deal nice burst damage and most things are dead before they get to you.
The Guardian, at this time, cannot have a Longbow as a weapon. In Beta, a developer mentioned openness to the idea, but it wasn’t a feature they could implement in time for the release or the near future.
There is no “best” for offhand and mainhand. You have three options for the offhand: Focus, Torch and Shield.
The recent patch gave Shield a small buff in PvE, now that Shield of Absorption can be detonated to heal. Torch is a potent aggressive offhand weapon for spreading Burn and removing conditions from allies. I don’t have much experience with Focus, but it offers Ray of Judgement and Shield of Wrath.