I played a very similar warlock in MOTB and I've used the Walk/Plague combo throughout the game (until the enemies became so weak that I didn't bother).
But different than you I opted for 3 levels of blackguard for divine shield, and in return screwed my dex to pump some points into power for power attack which is required for divine shield.
Despite popular belief, dex really isn't as substancial to a warlock, the only thing it really increases is the chance to hit with eldritch blasts, but that bonus to the blast-attack gets more and more irrelevant the more you progress with levels.
As race I opted for Drow because of max Char and Spell resistance. Between spell resistance, divine shield+invisibility, medium armor and bonus saves a warlock/blackguard really becomes a tough nut to break. For boss fights you might even get epic fiendish resilience, but that's really just an emergency button (btw, the telthor leg bone you can find early in MOTB is an excellent emergency button as well, especially since you can still use tenacious plague
).
Int is pretty good as well as there are some nice skills: UMD, spellcraft (gives eldritch mastery), Concentration, Intimidate, Hide (for Blackguard). Oh, and Drow gives a bonus to that as well.
I might try your suggestion of move silently.
