2) Will Trump be able to convince Boris to join him in this war, if yes will Boris be forced to delay Brexit?.
Not positive how it works, but I think if we go to war, NATO goes to war, and UK is part of NATO. I don't think any country could just say 'we're going to sit this one out'.
I don't think it will affect Brexit. Not all EU members are NATO members. I don't think the EU is capable of 'going to war'.
3) Does Iran have nuclear weapons and if they do have will they target it on US and their allies.
The Obama deal ensured that it would take Iran about a year to produce enough Uranium to be a real threat, and the deal ended mid 2018, so it's possible I guess, but that's just for the raw material so I think it's unlikely. At the time some people said maybe in a couple years, some said it could take them up to 10 years to develop the technology.
4) Why aren’t the world leaders trying and getting both countries to back off before a full blown war starts.
Some probably are. I wouldn't blame them for doing it privately in Trumps case. Doing it publicly could easily backfire. Turkey and Saudi Arabia share borders with Iran and Israel isn't far, I would imagine they are all working closely with our intelligence.
I wouldn't be surprised if Putin was also trying to calm down Iran right now.
If we go to war with Iran there will be no real winners.