First I tried to give a rough timeline of events about each member (leaving out Rei's entire story from DC). As there hasn't been much new background information revealed yet, most of the timeline refers to events mentioned in DC but I'll try keep updating it once more is revealed in WPS.
Hiromitsu & Rei
After Hiro's parents were murdered when he was a child, he was taken in by relatives in Tokyo while his brother Taka'aki (Komei) grew up in Nagano. They stayed in contact and Hiro told him about a new friend he had made in Tokyo which was Rei, nicknamed Zero. Sometime during high school he met up with Komei at a café and introduced him to Rei. He continued to keep in touch with Komei and wrote him another letter after graduating from police academy and officially becoming a police officer.
Matsuda & Hagiwara
When Matsuda was still a child, his father had once been falsely arrested which was his initial reason for wanting to become a police officer and joining the academy later on.
After graduating from police academy (supposedly in the same year they enrolled) he and Hagiwara were quickly assigned to the Tokyo Police bomb disposal squad. Shortly after, Hagi then passed away in the first series of bombings on November 7, seven years ago due to the bomb's countdown starting again after it had been stopped by the culprit seconds before detonating.
Matsuda, who had managed to disarm the other bomb that same day, was then transferred to the Robbery-Homicide Department 1st Division 4 years later with Sato as his superior. Just 6 days after transferring, he sacrificed himself to reveal another bomb location in the second series of bombings by the same culprit who had placed the bombs 4 years prior on the same day, November 7.
Unlike the other members who stayed together in pairs after graduating, Date became a regular police officer in the MPD Criminal Investigation 1st Division and seemingly lost contact to them for a while. Instead he was later assigned mentor of Takagi with whom he became good friends. One year ago, he then died in a traffic accident after a stakeout with Takagi on the day he was going to visit his girlfriend Natalie's parents and propose to her. After hearing of his death, she ended up committing suicide as well.
References & Details
Below I tried to include a list of references and other small details in WPS which have been shown or mentioned in DC before.
Rei being the #1 student of the academy. (WPS#1 and mentioned in DC#807 by Date)
His lack of teamwork in DC can be seen throughout the chapter as he likes to do things his own way, refuses help and guidance from superiors like Sato and ultimately decides to deal with the bomb on his own.
This is an interesting one which many people might not have picked up on. Because his father was falsely arrested as a result of improperly gathered witness statements he makes a remark on it in WPS#1 and, more importantly, he proceeds to interview the witnesses very thoroughly in DC#369 which caught the attention of Sato.
His exceptional shooting skills were also a major point in Movie 2 and were later confirmed to be canon.
Another more subtle reference is Rei fighting Matsuda here to find out why he hates him for being infatuated with the police which is further explored in the following chapters. While Rei doesn't admit it saying it was just answering a challenge, he fought him because Matsuda actually sparked his interest and he wanted to reconcile with him which they later do. Additionally, Hiro is like Elena here treating Rei with band-aids and wanting them to get along.
For theories regarding future chapters of WPS I've looked at unresolved subplots and undeveloped character traits to find potential areas which could or should be expanded upon in the future.
In terms of unresolved subplots we have the "Nagano Family Bloodbath Case" regarding the murder of Hiro and Komei's parents. To me this seems like the most likely story to be explored in future chapters as it has shown to deeply affect him and him researching the case to gather more information points towards it as well.
Hiro's parents were murdered.
Matsuda's father was falsely arrested.
Date's father resigning from the force still troubling Date to this day.
This is either just a coincidence or Hagiwara and Rei might have a tragic family background too that we don't know about yet.
This is more of a neat detail but Rei and Matsuda also both specialize in boxing so perhaps Matsuda could have taught Rei a thing or two he learned from his pro boxer dad.
Finally, a couple of minor things mentioned or shown in DC that have yet to be touched on include...
Matsuda teaching Rei how to defuse a bomb. (Mentioned in Movie 20)
Hagiwara also becoming very proficient at defusing bombs.
Date's trademark toothpick he's always seen with in DC.
Matsuda and Hagiwara starting to smoke either during or shortly after academy. (Shown in Episode 304)
That's all I was able to gather from the three chapters mixed in with stuff we knew from DC for now. Feel free to point out anything noteworthy I might have missed or share your feelings on the series as a whole. I've been thoroughly enjoying this spinoff manga and since we'll most likely have a little three-part arc about each member I'm excited to see where it'll go!