I can only give a partial answer, I hope that others can fill in the missing pieces.
Can this CHDK help me to raise the maximum time in aperture mode when the camera is controled over USB?
- remote capture (the official Canon one)
- an auto-exposure mode (such as Av) with CHDK
the answer is no. You can, however, switch to M mode when pictures become underexposed.
When using the CHDK PTP interface, scripting can be used to set the desired exposure parameters (correct exposure has to be calculated in the script).
If so - how to be sure it will be automatically re-activated after powerdown? The camera would be on some locked place, so the physical access will be very limited.
CHDK will load when using the bootable card
method. Starting the camera is another story:
- you can physically push the ON/OFF button
- permanently, then controlling the camera's power supply
- using a servo
- you can solder wires to the button and control it from outside
Some newer Canon PowerShots do not support the remote capture at all (for example PowerShot A800). Is there any hack for those cameras to support remote capturing (capture control over USB)?
Yes, using the CHDK PTP interface. For a start, see http://chdk.wikia.com/wiki/PTP_Extension
Note that AFAIK current gphoto also supports remote shooting on cameras that run CHDK (can't give details due to zero experience).