The coughing could be "kennel cough" and I highly recommend a vet check on that. While in there, have them check the leg.

The leg could be a bad landing, but that would be like a sprain and should have healed in 3 to 4 days.

Chicken nuggets, blech! I don't eat them, wouldn't give them to my dogs. I boil chicken breast (no skin, no bones) and use that as a really special treat for once or twice a week and at training sessions.

Banana is fine for dogs! Just not alot at once, it may either constipate them or give the runs, I can't remember which sorry.