What is the maximum safe value of bitrate or quality property?
You will have to determine that through experimentation for your camera.
Why it is impossible to improve quality of video?
Video quality is a function of many things - bit rate only matters if the camera electronics cannot transmit video information as fast as it processes it. Simply increasing the bit rate will make no difference if the camera does not process the video differently to take advantage of the faster bit rate.
This restriction of the program(firmware) or CHDK or camera electronics?
The camera electronics.
Whether it is possible to install in IXUS 105 a firmware from IXUS 130 or IXUS 100? In them there is support HD video. Electronics in IXUS 105 seriously differs from its "brothers"(IXUS 130, IXUS 100)?
There is no chance that you can take the firmware from one camera and load it on a different model. Zero. None. Nada.
The only way to really get higher quality video is to buy a better camera. Sorry.
