All are correct.
I'd put "themselves" at the end of the sentence (#5) if I had the choice.
I,1. We built the whole model (ourselves).
R,2. I knew the dog had fleas because it was always scratching (itself).
R,3. She (herself) is the owner of the gas station.
I,4. You boys will have to ask (Yourselves) that question.
I,5. They (themselves) gave us the good news.
I,6. We (ourselves) found homes for the abandoned kittens.
R,7. I decided to try to score the winning goal (myself).
R,8. Without oxygen, life (itself) would not be possible.
I'd put "themselves" at the end of the sentence (#5) if I had the choice.
2. R, I knew the dog had fleas because it was always scratching itself. (R) - Reflexive
3. R, She herself is the owner of the gas station. (R) - Reflexive
4. I, You boys will have to ask yourselves that question. (I) - Intensive
5. I, They themselves gave us the good news. (I) - Intensive
6. I, We ourselves found homes for the abandoned kittens. (I) - Intensive
7. R, I decided to try to score the winning goal myself. (R) - Reflexive
8. R, Without oxygen, life itself would not be possible. (R) - Reflexive