Hard to say. PG himself is pretty inconsistent until the playoffs. OKC is talented enough to compete with the top dogs, but lack the coaching to integrate the pieces. So if Donovan stays, then PG is gone IMO.
No, they aren't. They get nothing from the bench outside of Grant, and sometimes Abrines and Felton. They can compete with the big dogs if the big 3 plays well and they are defending at a high level (they are a good defensive team).
Melo is

...
Donovan was a prolific coach at Florida with two National titles and four Finals 4's, the man can coach. It's not his fault he's coaching an uncoachable player in Russell Westbrook who monopolizes the teams possessions, it's not his fault PG is streaky, it's not his fault Melo isn't the floor spacer he was supposed to be, it's not his fault they have no depth or bench, it's not his fault Andre Roberson got hurt.
Nikkas ignore how Donovan was outcoaching tf out of Kerr until the Dubs made adjustments and Durant/Westbrook failed to close the series.