I think the bigger issue is that England are playing warm up games in conditions totally alien to what they will experience in the WC. Possibly, with NZ in the middle east and Aus playing SA, they couldn't get games there...
They are playing the tri-nations tournament next up - I would think (hope) that Hales will have a chance then, or preferably sooner. He has gone big in the big bash there, but there is the thought that Hales struggled against India (spin) in the ODI series in the summer.
England selectors are traditionally conservative and risk averse. They possibly haven't seen enough from Hales in ODIs in the summer to put him in against spin-friendly conditions and/or without a spin threat, they want to have six capable bowlers (4+Bopara+Moeen). I accept that Cook shouldn't be in on form, but suggesting dropping him isn't gonna happen - we should speculate on who will play with him in the side, otherwise its fantasy cricket selection.
I think Bell's under the bigger pressure. If he struggles against SL, we may well see Hales in for Bell and Moeen at 3. Weird that Bell averages 45 since opening and was dropped down for Hales, yet stays at 3 when Moeen comes in? Bell has suggested England need to be more expansive, I hope he does. He's got the ability and I hope he watches players like De Villiers, Amla, Jayawardene, etc. and wonders why he isn't scoring ODI runs like they do. Its a conservative mentality, a British trait...
I'd pick Hales and pick Moeen instead of Tredwell. I do think we need to follow the Aus/SA model of throwing the hitters in knowing they will go big/have an impact 50% of the time. Trust Moeen and Root, who may go for runs, but a batting top eight of Cook, Hales, Bell, Root, Morgan, Buttler, Bopara, Moeen, with hitters like Stokes and Jordan to come should be capable of getting 300 runs.