Hello Jaap,
Photo Mechanic's Find and Replace utility from the Edit menu can search for a complete sentence or part of one and then replace it with your new text. If the line you wish to change is different in some or many of your images, then you will need to perform the Find and Replace operation for each different line in the Caption field.
Photo Mechanic will not be able locate the second sentence in the Caption field and replace it with you new target sentence. I hope this helps work workflow and please let me know more about your Rule 1 & 2, if you still have questions.
Best regards,
--Bob