Audio inputs are normally AC coupled (there is usually a series capacitor to block any DC component in the input). Typically this means you won't see much below about 20 Hz and you certainly won't be able to measure DC or slowly varying signals.
As for sensitivity, typical "live level" inputs expect a signal of 775 mV RMS which corresponds to 0 dBV. Microphone inputs are usually more sensitive than this, but there is no "standard" sensitivity and the input hardware often has some kind of controllable gain stage prior to the A-D converter.
This is a no-brainer, use the functional layout approach. There are a lot of bad schematics out there, including professional ones, so you will see pin order schematic layout sometimes. However, it is a bad idea.
Pin-order symbols is mostly laziness on the part of someone defining the part in the CAD system. Slapping everything down in the pin order is easier than digging out the full names and functions of each pin. Of course this isn't usually the stated reason. The most common excuse is that it aids in debugging. However, a little thought reveals that is not so.
When you are debugging a new board, you have both the board and schematic in front of you. Think about the usual work flow. Which is more common: "I want to look at the clock line, which pin is that?", or "I want to look at pin 5, which function is that?"?. Clearly the answer is the former, by a lot. Yes, occasionally early in the debugging process you may want to go around a IC and look at the signal on every pin, but that is usually once, if at all. There are cases where pin-order helps, like for some repair work, but for every one of those there are multiple cases when function order is better. Functional pin depiction is actually better for debugging than pin-order depiction. Don't get taken in by the excuses for not spending time on the symbol definition once.
Then there is the other considerable issue of schematic clarity. Here there is no contest at all. Pin order obfuscates the circuit and either forces a lot of air wires or forces other blocks to be put in inconvenient places.
For more on good schematic practises, see my more lengthy writeup on the subject.
Best Answer
The other 2 pins are usually normally-closed connections for the pins adjacent to them; plugging in a jack moves the adjacent pins away from them, breaking their connection. They are used either as sense or to shunt audio to internal speakers when no headphones are plugged in. OTOH, the fact that they aren't given in the schematic may mean that they are only for structural support.
A circle at the end of a wire indicates an external connection point where another wire may be connected either for some utility purpose or to make it part of a larger circuit.