Thanks Allbest,
Regarding point 1., I will again try to recreate the circumstances in which camera hangs, then I shall report.
Re. point 3., I have used all parametees in User AutoISO settings. Still it does not go beyond 1.6".
Re. point 4., what I have mentioned is real ISO which were converted with "Auto CHDK to EXIF v1.2" and recorded in exif data. For s3is the ISO range recorded is from 39 to 1499. I have not checked with histogram method (it was mentioned elsewhere in the forum) as I am not very familiar with this method.