How did Nero know where and when to find Spock Prime? I know that his 20+ year absence can be explained by the Klingon prison term in the deleted scenes, but I'm still curious as to how he and his crew were able to apprehend Spock.

Math?

Well, he's had 25 years with nothing to do but calculate that.

but what happened with his ship while he was detained? Wouldn't it be possible for Spock to pop up around then just as much as the exact time he did?

to my understanding from what was shown (and I could be wrong) didn't Nero literally just wait around for Spock to come to that exact location hoping that eventually he'd pass through? If anything I would've kept in the Prison scene somehow...but I can't see how cuz it would kill the pacing early on

After the battle between Nero and the Kelvin, the Klingons attacked the Narada and took her crew prisoner. They were taken to Rura Penthe, where they were held prisoner for about twenty years.

While on Rura Penthe, a former Starfleet officer named Clavell told Nero that Spock and his ship would also have gotten pulled into the black hole along with Narada, but that he would not be unable to accurately calculate where or when Spock would re-emerge.

At some point, the Narada "awakened" and broke Nero and his crew out of Rura Penthe and destroyed the Klingon fleet. The Narada, operating on its own, flies to near the Delta Quadrant, zeroing in on an energy source.

The Narada arrives at a nebula, and Nero enters it. The "nebula" is, in fact, V'Ger. V'Ger could sense the Narada as sort of kindred spirit and sought it out. Nero is able to have V'Ger calculate the time and place Spock would emerge in the past.

Nero is then waiting on Spock when he arrives, takes him prisoner, steals the red matter (ie, Rambaldi device), and maroons Spock on Delta Vega.

Seeing as how Nero and co. were in a time period where Romulus had yet to be destroyed, couldn't they have just flown there and warned their fellow Romulans? If memory serves, they (the Romulans) were not on the best of terms with either the Federation or the Klingon Empire at the time, so they most likely would have welcomed a motley group with advanced weaponry and a bad ass ship. Instead of destroying just Vulcan, Nero could have formed an armada and set out to conquer the whole galaxy. Also, how does Kirk manage to get marooned on the same planet and just a few miles from Spock prime? Coincidence as a plot point really bothers me.