Someone is unsure whether he ate a half-log of impure food (the Rabbinic minimum). Is he impure?
Halacha 1: a doubt about whether one performed an act that causes Rabbinic impurity (like eating impure food) is ruled pure — doubtful Rabbinic derivative impurity is always pure.
Question 2
A weasel with a neveilah in its mouth passes between pure terumah loaves. Doubt whether the loaves were touched. Are they impure?
Halacha 7: when a weasel or dog carries neveilah while moving, if pure things pass between them (or they pass by pure things), the pure things remain pure — the impurity had no fixed location.
Question 3
Three people are in a private domain together and a doubt about impurity arises. Is it treated as private or public domain?
Halacha 2: the sotah parallel involves two people (suspect wife and the man). Extending this: when three people are present, there is no longer a one-on-one relationship, and the doubt is treated like a public domain.
Question 4
A blind person in a private domain is in doubt whether he touched impurity. Is he impure?
Halacha 3: a blind person, sleeping person, or one walking at night — though they didn't see what happened — have the capacity to respond to questions about what they did. Therefore private-domain doubt = impure.
Question 5
A sheretz was found placed on top of a loaf of bread after being thrown. Doubt whether it touched the loaf. Is the loaf impure?
Halacha 6: the thrown-sheretz rule follows the status at discovery. If it was found not touching the loaf (to its side), we don't assume it touched and rolled away — it is pure. All impurity doubts follow the state at time of discovery.