Some thoughts, in no particular order:
Assault Swordmage doesn't really have any synergies with a Con Lock. They use Strength for all their riders. Shielding Swordmage, however, uses Int/Con, which is perfect for an Infernal or Vestige Warlock. You only have to worry about 2 stats, and put your extra points into Charisma.
Dark pact is based around Charisma attacks, which you shouldn't really have to worry about. In fact, its actually a pretty sub-par choice, as going for the Dark Pact at level 1 gives you a practically useless at-will, and no pact boon.
I'd suggest getting your Swordmages Warding feature from Hybrid Talent. Its pretty much a necessity for anyone who's even close to a defender.
If you plan on going Infernal pact, consider combining Hellish Rebuke with your Shielding swordmage Aegis for awesomeness: Attack me, take damage. Attack my allies, deal no damage.
Minor actions for you will be severely limited. You'll be using them on your turn to either curse or mark things. For that reason, I would suggest taking swordmage powers that apply your mark automatically. There are also feats that allow you to place your mark off-turn as well.
My suggestions:
* Get both an 18 Int and 18 Con after racial bonus, with 12 in Dex
* Take Swordmage Warding as your Hybrid Talent for extra defense.
* At level 11, retrain out of your Hellish Rebuke (pick Vestige or Sorcerer King Pact as your main pact), then pick up the feat Twofold Pact. This will give you the at-will and boon of a second pact. Pick Infernal. You now have the Hellish Rebuke and Dark One's Blessing.
* Your Dex will be 13, pick up a multiclass assassin, and then Cursed Shadow.
This assumes you'll make it to level 12 or 14, so you might have to adjust this if you know you aren't. Hybrids are feat intensive and you have to know what you're doing, else they become pretty gimped compared to regular characters.
If you're going for just for the "melee mage with a sword" theme, look into making a pure Hexblade. They pretty much get the whole flavor of what you're going for, and are a bit more mechanically sound. You could even multiclass assassin or rogue for some extra sneakiness.