There is nothing wrong with 250kts downwind if you expect to go far enough downwind to generate enough track miles to slow down and go down in good time. There are numerous factors to take into account when judging the appropriate speed and altitude for any given position of the aeroplane, be it downwind or anywhere else (except of course below 1000ft when you're either on profile and speed or else in a go-around). We don't know how many miles this FO expected to have (except that it was clearly more than transpired), so we don't know whether he was right or wrong to be 250kts downwind.
He may well have been too fast and too high for the expected track mileage, in which case the skipper should have intervened earlier. But he didn't, and as no pilot is immune to the laws of physics, he should have realised that you play the cards you are dealt and refused the base turn until sufficiently slowed down and lower. End result: a stable approach and a discussion on the ground about an appropriate downwind speed for the conditions of the day.
The rest of it was a balls-up. Ignoring the mandatory stabilised gate of 1000', particularly as your company doesn't have a 500' vmc gate, is unforgivable. The go-around attempt by the FO was the correct decision. Some approaches just don't work out, it's no shame to GA, try again and discuss it all on terra firma.
Stabilised at 300' is not stabilised. Any captain that allowed the aeroplane to get into the situation it was in (regardless of who was flying) is negligent. The buck stops in the left seat, which gives a skipper the obligation to fly out of trouble, not the discretion to fly into it.
Flaps 15 not on speed, too high, no landing checklist completed.
This is unstabilised, pure and simple so
You don't specify the runway length, nor your touchdown point, nor whether stopping the airplane on the pavement was a violent maneuver. There is a lot of maneuvering latitude during a visual approach before it may be classified as "unstable."
is irrelevant. Unstabilised at 1000' (if that's the company SOP), GA, try again and do it right second time. Going below in the hope of recovering the situation is not an option, at which altitude do you decide, enough is enough?