about the real career of waid

Contrary to what the movie will have you believe, the WBU title is not a "real" title. In boxing pretty much everybody can decide to start a boxing organization and make "world title" fights. There are literally dozens of organizations, but only four (WBA, WBC, IBF & WBO) are really recognized.
The WBU title Micky Ward won didn't really matter to anyone within boxing and Shea Neary who Ward got it from was never a world-class boxer and held the title basically as a marketing gimmick to promote his "world title" fights for people who didn't know better, as the WBU had a short spell of popularity in Great Britain at that time.
Ward probably never defended it because it meant nothing.
After all, Ward had fought for a real world title 3 years earlier (right after the Sanchez fight that's in the movie), but had lost in 3 rounds against Vince Phillips.

The reason he only defended his WBU Intercontinental title once is (apart from the fact that again it's the WBU) that an Intercontinental title is used as a stepping stone. In boxing you can't hold a world title at the same time as for example an Intercontinental title. Many boxers start out with winning national titles, then vacate it to win a International Title or something similar and then vacate that to go for a world title. Therefore those minor titles are usually not defended that often, apart from when you think your boxer has to fight a few more fights at that particular level before stepping up to bigger titles.

And splitting his next four fights simply means he won two and lost two. He split the four-fight-series, so to say.

Hope this was of help.