For basics, Beguiler 1/Wizard 4/Ultimate Magus 10 (or however far you get) is by-far the best choice. Beguiler 2/Wizard 3 as entry is not going to get you killed or anything, but it is a dramatic loss of power relative to only a single lost wizard level.
Cloaked Casting
From an optimization perspective, Cloaked Casting isn’t nearly worth what you lose in terms of wizard spellcasting.
Unfortunately, Cloaked Casting is rather minor in general. There’s no reason you can’t surprise your opponents with your spells, and just not get the +1 DC bonus. Surprising people is usually a pretty good idea anyway (though feinting in combat is not). You will be better off doing things this way.
But feel free if you really want Cloaked Casting to get it. Just be aware that it will cost you more than it ought to.
You don’t have to ban Illusion if you don’t want to
Wizards get illusions that beguilers don’t, and your wizard spellcasting will be superior to your beguiler spellcasting. If illusions are important to you, keep it on your wizard; having it on both classes will really play up its importance to your character.
So illusion could be very valuable to you; don’t ban it just because you think it will be redundant. It won’t be if you use it right.
You really don’t need area-damage
Conjuration and illusion are excellent at battlefield control, which means rather than simply damaging people in areas, you’re hitting people in areas with nastier effects, and stuff that warriors cannot do. Damage is easy, and lots of classes do it as well, if not better, than spellcasters. In a lot of cases, though, that’s all they can do. So let them do it.
And ban evocation.
Metamagic
Go for it! Ultimate magus gives you powerful abilities that allow you to use your second class to power metamagic for the first, which works very well. Definitely a good choice.
Shadow Spells
Ideally, you do not want to use shadow evocation or shadow conjuration offensively, because you give the enemy two chances to save, which is bad. As such, I don’t really suggest relying on them for that; they’re massively better as buffs or what have you. Spell Focus (Illusion) is still a solid idea though.
There isn't an officially stated lifespan for Changelings (as of writing this answer).
The most important thing to remember is that Unearthed Arcana material is only effectively playtest material. It's not "officially" published or polished and thus isn't nearly fleshed out as much as the core rule books or supplements so you're not likely to find that level of detail until it's actually put into a supplement book, if it even does.
There's only one other mention of changelings that I can find and that relates to doppelganger children in the Monster Manual but even that doesn't give us any information on their lifespan.
However, if you really want something to base it off and get a rough idea then it has been detailed in the 4e Eberron Player's Guide (unfortunately I can't speak for older editions):
Changelings reach maturity at about fifteen years of age, and they live about as long as humans do.
Best Answer
No, you’re not really missing anything
With regular specialization, as long as there is at least one spell from your chosen school you want to cast each day, you can do that and use your regular slots for any other schools you might be interested in.
Dual specialization only helps you if you cannot do that, and have some spell level where you really don’t want to cast any spells from the school you’d choose first.
Which might happen, maybe, sometimes, but not often; certainly not often enough to justify banning a third school.
Changeling wizard? Recaster!
Seriously, recaster from Races of Eberron is awesome and you should seriously consider taking it. One of the very few cases where losing a spellcasting level is justified. It’s flavorful and powerful.