Re: "Shows value of having 9 engines"
I don't think they have said exactly but I believe that there are some differences between the engines so specific ones are required. e.g. not all can relight and possibly not all can throttle down low enough. I think the centre one is pretty much essential at landing time.
If you watched the re-entry burn there was a lot more swinging around than normal which suggests that the problem engine was either one of those 3 or had perhaps been damaged by the explosion(*).
(*) - If you watch a few seconds before the main engines shut down it certainly looks like something exploded - it was a bit fireball-ish...