Yeah so this was the offensive team with dual screen support, but with a two-punch core, I thought it was screens and + 5 sweepers. Anyways, the only tweak I would probably recommend is a bulky Porygon spread. I'm thinking somewhere along outpacing base 80s or base 85s and dumping the surplus to HP. 40 HP / 252 SpA / 216 Spe with a Timid nature or 80 HP / 252 SpA / 176 Spe. With screen support, the stuff that will be outpacing you will be doing little damage to you for the most part. I don't think you'll be losing much from this as most threats faster than you are handled by your one-two punch core. With Milotic's EVs, I don't think Timid Moltres will be a huge problem. Modest versions are outpaced.
I would defenitely keep the Curse set for Registeel. Without screen support its deadly enough as it is and with support I could imagine it being a mission to take down. Like you said, Cresselia/Milotic handles most stuff that troubles Registeel. The only evident change I would go for is Earthquake over Sleep Talk. Allows you to strike Houndoom, Blaziken, and Arcanine for super effective damage. Now the best idea would be going to Milotic, but in certain situations where you must attack, it will come in handy. As an added bonus, you can hit raikou hard too (something difficult I would imagine without dual screens down).
As for other options, reverting to taunt on Mismagius would probably be best. Allows you to effectively combat with stall teams and prevent someone like Cresselia from further boosting its spd stat. But most importantly, it prevents Clefable and Chansey from ruining you with Encore or Thunder Wave respectively. I have used a similar team like this and found that Encore is a problem to Cresselia, Porygon, and Mismagius. Taunt will allow you to swing the momentum to your favor and it scares away Encore users. Although take note dat your prone to Sucker Punch users, but thats the only exchange for handling stall teams mainly. With Reflect support, Im pretty positive Mismagius can survive. overall gl.