Does the marriage contract become valid again if the wife mistakenly believed she had looked with lust at her husband's father, which would necessitate perpetual prohibition of marriage (tahrim al-musahara), and then she confirmed that it was merely a delusion and not a look of lust?

The wife is not forbidden to her husband if she looks at his father with lust. The Hanafis, who believe that looking with lust causes prohibition, restrict this to looking at the private parts specifically, not the rest of the body. If the matter is merely a whisper (waswasah), then no attention should be paid to it.