CovertOps, you're confused. If you use Novice Power to exchange your 3rd level encounter power for a different class's 3rd level encounter power, you aren't stuck with that other class's power forever, or even until 13th level (when you can replace your 3rd level power with a 13th level one). As per page 209, each time you gain a level, you can change which power is traded for the other class's power, as long as the selected power meets all the requirements.
Suppose you're a rogue multiclassing wizard.
You take Novice Power at 4th level. You can trade your 1st level encounter power for a 1st level wizard encounter power, or trade your 3rd level for a wizard 3rd level power. Suppose you trade your 1st level slot for Ice Storm.
When you level up, you can change that choice. You could decide to give up your 3rd level rogue encounter power and take Icy Rays instead, and you gain a level 1 rogue encounter power in place of Ice Storm (because you're no longer using Novice Power to exchange it).
At level 7, you can choose to take a brand new rogue power... Or, if you prefer, you can take a brand new level 7 wizard spell, at which point you lose the 1st or 3rd level spell you'd taken earlier, and replace it with a rogue power of the appropriate level.
Basically, at all times, you have one encounter power traded for a wizard spell, but it doesn't say anything about what level that traded power has to be, so long as you give up a rogue power of the same level.
To the OP:
It might help to think of this in terms of slots. When you reach 13th level, your level 1 encounter power slot turns into a level 13 encounter power slot. Once that happens, you can decide what power to put in it -- you can pick one from your primary class, or you can put a secondary class power in it with Novice Power.
Following the previous example, you might still have Ice Storm in your level 1 slot. You lose that level 1 slot and get a new 13th level slot. You can take a 13th level rogue power and replace your 3rd or 7th level encounter power with a wizard spell of the appropriate level, or you can just put a 13th level spell in that new encounter slot.
If you'd had Icy Rays in your 3rd level slot, you'd lose your 1st level rogue power and then you'd have a new empty 13th level slot. You can (again) pick a rogue power, or put a wizard spell in there. If you put a spell in there, you lose Icy Rays and gain a 3rd level rogue power in its place, because Novice Power is now being used on that 13th level slot.
It's a lot more complicated to write than it really is in practice.
On the half-elf thing, you can change which power your Dilettante ability is giving you by using the Retraining rules, but there's no level involved there. You always have to pick a level 1 at-will attack power. You can decide you don't like Thunderwave and swap it out for Tide of Iron, but you can't ever get a power that's higher than level 1.