1. If the strap button is level with the 12th fret (or closer to the nut), and if you tuners are not too heavy (particularly on 5 and 6 strings) the bass should balance nicely with your arm on the upper bout. The other thing is, your bridge acts as a counterweight to the headstock. A modern high-mass bridge and a couple of humbuckers helps things a lot.
2. As deep as you can. I'd say 25mm. That way you have 5mm for the front and 5mm for the back. I like to route as much timber out as possible. My my latest 6 string... Build No.8 - Recycled multiscale "coffee table" bass
3. How long is a piece of string? What the timber would done to the tone unchambered is anyone's guess really.
What chambering effects more than anything is the 'density' and therefore the 'stiffness' of the bass. By removing the timber, it makes the bass less dense, a little more flexy and hence more 'resonant'. This robs it of a smidge of sustain, but lets face it, bass guitars are not usually lacking in sustain.
You may have seen Roger Sadowsky's style of chambers (lots of small bean shaped holes). This was done in order to keep the swamp ash body slabs more consistent, lighter and resonant as he found the wood was getting heavier. He and his customers are convinced it works.
Whether you use his method or just hog out big chambers, it does effect the structure, therefore the sound. In the end, I chamber for weight and the hope of extra resonance. I think it works, others will think otherwise. I think I win either way, because less weight (with proper balance) makes for a more comfy instrument.
Hope that helps.