hello, Sony Batt are Li-ion.....

yes, all rechargeable battery cannot be 100% flat. once it become zero, the battery is dead, only with special device or high end charge 'may' short it to bring it back to life (no guaranty).

shooting for 5 years, never removed it. but will always check the % before leave house.
the Zig Zag is normal. the sound is noticeable with all my lens, on the 35, the aperture is much bigger then other lenses.

i. the sound is due to wider aperture lens, F/1.8 is a really wide aperture. the 'distance' for the aperture to close down from F1.8 to F4 is more then F3.5 to F16, hence the distances means louder and longer obvious noise.
DSLR/SLT aperture create the same noise too, just that mirror flaps and shutter sound is too loud that you wouldnt able to heard it.

ii. why there is a zigzag to begin with? it only happens when you focus... the aperture will open widest it can to focus because it will need all the light it can get to focus. and will close back down to the aperture you are on (eg, F4), and if you notice, when closed down, the Liveview will show a little more noise in darker environment due to the light gain on F4(or darker) for liveview. DSLR only closes the aperture during shutter release because if its close down full time, imagine a F8 optical viewfinder, its going to be really dark.

but if you shoot wide open like F1.8-F2.2, you will hardly hears it.

hi bro, how much u got it from? =)