but "free amnesty stamp" not exist, it mean you will not get any stamp before 30.04

In my opinion its true. My tourist visa ended 15 april, so in that day I went to immigration at CR for information whats next, and woman told me that I am safe to 30.04 and after that I can normally extend my staying for 30 days for 1900 THB (if its your 1st time), of course if amnesty will be extend - you still have your "extension" after amnesty will be over, this is what she told me.