Why do you see jumps on the Internet and at times the quality is bad? 

Internet viewing has nothing to do with the actual local recording quality (in the vehicle). While in the vehicle you can record in D1 quality and at 15 images per second; Internet is recorded at 3 or 4 images per second and at a CIF quality (current maximum).

The transmission channel is 3G upload; with very limited speeds (compared to fiber or ADSL) therefore speed and quality is lower; also when changing antennas, coverage and bandwidth there may be small stops as well as changes in quality and speed. 

Can I take advantage of the reversing screen I have on the bus to view the cameras?

If the screen has two video inputs and one is not used (something usual), it can be used. 

Can I place hidden cameras and record audio? 

In the EU not (due to the existence of a specific European data protection regulation) but in the other countries it is usually allowed. National regulations must be reviewed.

Is it mandatory to place the "Video Surveillance Zone" signs inside the bus?

In Europe it is, in other countries it depends on national regulations. However, it is usually recommended and very common.

What guarantee do the teams have?

2 years warranty