I hope a516 doesn't mind, but I can have a crack at a few of them....
1. USA Direct was a News 24 programme so my guess is that World wanted to show it too and decided that Newsnight was surplus to requirements? I know for certain that Newsnight was not shown on BBC World by the year 2000 when I first started watching the channel.
2. The .30 news summary on World is a fairly recent invention. Setting aside The World Today, which, as you rightly say, had a half past summary, I don't think it was rolled out across the channel until the red/black era - although which version of that I can't remember. My guess would be some time around 2004/05? It may well have been when the channel starting 'streaming' in 2004. That was when each bulletin had two ad breaks, some of which were taken by various regions and others had the newsreader filling in between stings. Prior to 2004 there was only one ad break per bulletin (and sometimes no breaks at all) and the presenter was off air in between the stings. If your region didn't show ads, you got the breakfiller. This became very repetitive, hence the decision to start 'streaming' the channel.
3. BBC World and News 24 have simulcast many, many times over the years. This has happened for set renovation or something as mundane as studio/gallery maintenance. There are just too many occasions to recall them all.
4. BBC World started showing some programming in 14:9 when the channel moved into N9
in 1998. Prior to that it was based in N1 which was only 4:3 capable.
I hope that answers some of your questions - but I'm sure a516 can give some more detail.