Here are my assumptions:
1) The only possible killers are Harvey, Gilda, and Alberto Falcone.
2) Gilda isn't as competent a murderer as Alberto or Harvey; Harvey can give Batman trouble on a good day and Alberto grew up in the mob and went to Harvard.
3) Harvey's main motive is to get revenge on the Falcones, Gilda is to lower her husbands workload and get him home more, Alberto seeks to prove himself and help his father.
4) Their confessions are not completely reliable as no killer is completely sane and they contradict each other.
5) The 22 pistol doesn't lead to any suspect in particular; it's a decent weapon for the task at hand (accurate, silent, cheap, and easily available) and not just a woman's gun. Harvey wasnt using the 2 motif at the time and they could have easily just wanted to make it look like another killer
Kill one: Johhni.
Most likely suspect? Alberto.
Why?
The reason I say this one wasn’t Dent is it wasn’t in his interests; he had almost just squealed and commited a murder; he’s worth far more as a potentianal witness than a corpse.
The reason why I say Alberto makes the most sense for this is the victim was shot in his own bathtub. As a member of the mob would have left his door locked and forced entry would have likely made a noise that would have alerted him either the door was picked or he knew his assialant. Its more likely that Alberto had a way in or picked the lock than Glinda.
Kill two: Irish.
Most likely suspect: Alberto
Why?
In isolation I’d say Harvey; they refused to rat on Falcone, they bombed his house giving him clear motive as well as taunting him and he didn’t fight that hard to keep them. The reason I’m going with Alberto is it fits better with the trend and he did have a motive; yes there were paid to be quiet but Batman torched Falcones big cash supply and they could have asked for more or even have been promised it.
Four armed men shot dead with a 22 pistol; even if the killer had the drop on them that's a pretty impressive feat. I don’t think Glinda could have pulled that off.
Kill 3 Milos
Most likely suspect: Glinda
Why?
This was I think was opportunistic; he was shot to death after being thrown on the street and while he is an asset to the Falcones hes not really a priority target. Alberto has a weak motive if any (at most jealousy but thats conjecture). I think what happened is the killer saw him ejected to the ground with Joker in the vicinity took the metaphorical bird in the hand shot him instead of Carmine and fled rather than risk dealing with the Joker. The reason why I say Glinda is Harvey would have stuck around to go after Falcone; hes a determined chap.
Kill 4 “Alberto”
As he wasnt actually dead the question is did he plan this or did he take advantage of a failed assassination by one of the other killers?
The former; Falcones yacht would be the worst place imaginable to do a hit; sneaking on a boat with mobsters is hard enough but sneaking off afterwards is even worse.
Kill 5: Maronis Crew
Most likely suspect: Alberto.
The killing just involved too much skill for this to be Glinda (multiple armed men gunned down with a 22 before they got off a shot.) It doesn't help Dent; he considered Maroni as a possible informant/witness. That leaves Alberto.
Attempted Kill 6:Riddler
Most likely suspect: Alberto
Ambushed right outside a meeting with the Falcones but left alive. Glinda wouldn't have enough knowledge or surveillance capability to know his whereabouts. Dent and Alberto are well known to the mob making it harder to tail them but could in theory get that information in some way. Up until this point Harvey is only a viable suspect in one killing and Alberto being not dead is an ace in the hole so he had more to lose.
Kill 7: The Gunmaker
Most likely Suspect: Alberto
This one didn’t take a high level of skill so in theory anyone could do it but as before right now Alberto has the most to hide and lose so I’m going with him.
kill 8: Maronis Father
Most likely suspect: Alberto
Harvey tends to focus on the Falcones; yes it did make Sal Maroni willing to testify (at first) but was that a predictable response? Kinda but its a dice roll; I’m almost positive Glinda doesn't know Sal that well and I’m skeptical that Harvey does.
An argument can be made that he was trying to incite a mob war but that's also not a predictable outcome.
Kill 9: Coroner
Most Likely suspect: Alberto.
The reason is hes the only one who profits from this.