The PL season is fought out over 38 games - not the last one.
If Cardiff had won the game they lost who is to say that Brighton wouldn't have beaten Arsenal?
You cannot predict any results based on what ifs.
Of course Hughton is responsible for how the team played - he was the manager and the buck stopped with him. From Hughton's perspective he wanted to keep the team in the PL and he adopted the approach he felt was necessary to achieve that objective.
In my view Hughton is a very under-rated...
Brighton got enough points to avoid relegation - Man City got enough points to win the title. After 20 games Liverpool were 7 points ahead - using your logic you could claim that Man City only won the league because Liverpool were worse than them and blew it by losing three games they shouldn't...
And Man City wouldn't have won the PL if Brighton beat them on the last day of the season - league positions are determined by results - the results Brighton got and the results of the other 19 teams as well. Chris Hughton was the manager of Brighton and Brighton are playing in the PL next...