Okay, the first part about the uncle wanting to pay the tip, that's completely inappropriate IMO. The person in charge of the tip is the person in charge of the bill. If someone else wants to add MORE to the tip, they are always free to do so, but you cannot force the person paying the bill to tip less than they want to*.
Secondly, I won't speak for anyone else. But if I went to a restaurant and was treated that poorly by the waitstaff, if there were no other circumstances that I could see to explain it, I wouldn't tip as much. If it was absolutely swamped and there was only one waitress, she may just be extremely busy and not notice the drinks you need. Or remember exactly what they were.
The wrong entree problem had absolutely nothing to do with the server or the busboys, and their tip should not have been affected by it. That was a mistake by the kitchen. I will never penalize waitstaff for a kitchen mistake, and I will always give the waitstaff the benefit of the doubt. If it was fixed promptly by the manager, then I don't see a problem with it.
*My aunt is notorious for trying to control other people's tips. She and my uncle were treating all of uncle's siblings and their spouses to dinner one night. The service was wonderful, the food was great, and my dad wanted to leave an extra tip for the server in cash on top of whatever my uncle left. My aunt, his SIL, actually took the cash he set down and tried to put it into her pocketbook because, in her words, "That waitress didn't deserve that much extra." The audacity of not only trying to dictate if someone else offers extra tip money, but to try to take it herself, appalled me when I heard the story.