First, I think you should have a quick read of some general overview on battery voltage selection for your projects, and in general the power stage selection is important. I have written up an informative power point presentation that I delivered to the members of my university's robotics club. You can see it here link to PDF document about power systems for robots
The basic answer to your Q1. is given the requirement of both power rails, I suggest you use a buck converter with 1-2 amp output at 5V for the servos, and 'daisy chain' a 3.3V linear (LDO) regulator off the 5V rail for your digital systems. Use lots of capacitors to prevent brown-outs and other issues that can occur if your servos pull the 5V rail too low (if they get jammed, or you are abusing their range of movement too much).
For heat dissipation purposes, you could also put an LDO (5V, 1A) on the 12V rail for the servo, and have an independent supply for the 12V -> 3.3V of your digital system on another regulator IC. This method is better if you are stuck with just a few adjustable linear regulators - and safer by having the supplies independent of each other to avoid brown-outs due to the servo stalling. If you were to daisy chain the regulators, you may still have the drop-out issue, but mostly you should avoid pulling any more current than necessary from the 12V -> 5V regulator, for heat reasons. The low current 12->3.3V LDO should be fine by itself, but it's constant draw if daisy chained to the 12->5V LDO may be detrimental.
To answer Q2. I suggest you try to use some pre-made (cheap, from lots of places online) buck regulators to get your 5V and 3.3V supplies each connected to the battery. If you can only get one buck regulator, get it for the 5V rail and use an LDO for the 3.3V supply. Again, big fat capacitors to help under pulsed loads, such as a servo moving back and forth quickly!
I hope that helps. Please check out that PDF document I made, it may help you understand why, and it gives you some example scenarios for what to choose and when.