I think all of your questions were very good questions.
I just can´t play around with setting and maybe kill whole site.
BPS Pro would not kill your site. BPS Pro is designed so that you can easily and quickly reverse something and/or turn things on and off with one click: http://forum.ait-pro.com/forums/topic/read-me-first-pro/#bps-pro-general-troubleshooting
Actually BPS Pro is very simple to setup and use, but you are right that what BPS Pro is doing is complex in itself. php errors in general can sometimes be difficult and frustrating to figure out, but the purpose of the BPS Pro php error log is to let you know about coding mistakes/problems/issues occurring on your website and/or in other plugins and themes that you have installed so that you know they are occurring and what needs to be fixed/corrected.
If you do not feel comfortable posting in the forum then you can send questions/issues directly via email to info at ait-pro dot com.