I would say Isaiah but I am pretty sure he was with Portia when the shooting took place... and how he would have gotten the gun would be a stretch... He would maybe have done if for Portia.... and he was tormented by Sidwell which may have caused him to have PTSD or something he could use as an excuse.
And... we all know that Stella visits with Tracy at the Q mansion from time to time... so her getting the gun would be plausible... and she most certainly had plenty of motive.
Heck... Even Martin would be a good twist! He has put up with enough of Drew's crap for a long time.
I would be okay with it being Molly... Girl gets no respect... maybe she decided to take out years of pent up anger on Drew.... and who could blame her? Her daddy would defend her and get her out of it. Or they could have Cody step in and take the blame for her...
Danny would be interesting... if disappointing because it would somewhat justify Drew not wanting him around Scout. He has free reign at the Qs and has a history of doing what he wants.
There are SOOOOOO many people with motive... and the PCPD is just focusing on a few.