I was thinking about this too, so here's my build (added an additional Feat from your houserules):
Lvl 4 Tiefling, Archfey Tome Warlock
This build points to forgive the melee and AC options for a more focused caster. I liked your idea of an hybrid, but seems to me that it really needs to be an hexblade, going in the direction of the other classical builds around. Tiefling adds some interesting features, while allowing to reach 20 CHA on lvl 4, increasing the overall effectiveness and chance to hit of the few spells you can cast.
By Lvl 4 you have:
- Cantrips: Eldritch Blast, Minor Illusion, Vicious Mockery, Thorn Whip, Shocking Grasp, Thaumaturgy, Friends
- Lvl 1 Spells: Hex, Sleep, Faerie Fire
- Lvl 2 Spells: Hold Person, Phantasmal Force
- Hellish Rebuke 1\day (2d10)
- Fire Resistance
- Agonizing and Repelling Blast
I chose Archfey over Old One for several reasons:
- Spells: Dissonant Whispers and Tasha's seems kinda redundant to me, especially when you can get Hold Person at lvl 4. Sleep it's an
excellent spell vs. low level mass enemies, it has no concentration
and grows with spell slots. Faerie Fire can grant advantage to the
entire party vs one or more hard targets. Both pacts have Phantasmal
Force which is a great disable for low int enemies and fun to use too.
- Pact Powers: Fey Presence is an area disable which fits more the theme than Telepathy granted by Old One. At lvl6 Entropic ward is
fine, but Misty Escape is simply awesome.
Invocations: I'd go to PimpMyBlast just because this is your only reliable source of damage in a lot of situations where CC or forced movement are not options - If all CCs fails, if you want to blast that minion in the backrows not worthy a spell, in long fights, ecc..
But if you like, you can forgo one of the blast invocations to get rituals, which can give you a bit more flexibility and a Familiar.
Feats: i'd really like going first at all costs with this build, so i'll take Alert. A well placed Sleep or Hold person the first round is no joke. Otherwise Moderately Armored can give you better ac with a shield, but going melee will require War Caster too to be a viable option.
Ability scores in order of importance: Max Cha, High Dex, High Con, Mid Wis, Low Int, Low Str
Role: cc\secondary caster
Playstyle: Disable high treat enemies as soon as you can. Use forced movement (Blast, Thorn Whip) to generate AO from allies and reposition enemies at your vantage. Minor Illusion to modify battlefield (covers, false gaps, ecc), Vicious Mockery to help ally, Shocking Grasp to disengage, Thaumaturgy\Friends as social tool.
Progression: At lvl5 you can get the infamous Devil's Sight + Darkness combo which can transform you into a walking nightmare of disabling madness, very hard to target. Lots of tactics here, disabling sight with plain movement is a powerful option, plus you attack with advantage; just take care of LOS of your allies. If you want more utility you can always get Rituals. Darkness is 1/day from Tiefling so you can also get a lvl 3 spell. While patron's spells are not that great, at lvl 3 you have 2 strong area disables (Fear and Hypnotic Pattern) and Counterspell, which is really useful if you are the only caster in the party.
Here is Lvl detailed progression:
lvl 1
Tiefling: +2 Cha, +1 Int, Thaumaturgy
Patron Fey: 1-turn Area disable
Spells: 0 - Eldritch Blast, Minor Illusion; 1 - Hex, Sleep
lvl 2
Invocations: Agonizing + Repelling
Spells: 1 - Faerie Fire
lvl 3
Pact of the Tome: Vicious Mockery, Thorn Whip, Shocking Grasp
Spells: 2 - Phantasmal Force
Tiefling: Hellish Rebuke 1/day
lvl 4
Spells: 0 - Friends; 2 - Hold Person
Feats: Alert (from houserule 4th lvl)
Their use is unlimited. That's kind of the point of the Warlock - he gets very, very few "spells known" but he isn't limited in use of them. He's also not technically a spellcaster, but that's getting pedantic.
Complete Arcane page 7, Warlock class description, Invocations heading, emphasis mine:
A warlock can use any invocation he knows at will, with the following qualifications:...
Their invocations are infinitely usable, meaning that two warlocks with different invocations are completely different in combat. Two wizards with different spell selections could learn from each other and become more alike; two sorcerers with different spells known can't combine their knowledge, but they're still going to feel a little alike - whether it's a fireball or a dominate person they're holding back, they're still likely to stick to their smaller spells until a situation calls for them to fire off one of their game-changers. Warlocks don't have a single big go-to one-shot like other casters do, but their entire style is different - the warlock doesn't mind starting every fight with darkness spells and AoE attacks and charming the enemy's tanks, because he can still do it again... and again... and again.
Best Answer
I am assuming you are getting Sneak Attack from the Sneak of Shadows multiclass feat.
As you say SA requires you to use a light blade, a hand crossbow, a shortbow, or a sling. What it does not require is the attack to be a weapon attack, so as long as you are using one of those as an implement you can apply your SA damage. The easiest way to do this would be to take Arcane Implement Proficiency and choose light blades or if you worship Sehanine you can take Moonbow Dedicate to gain shortbows as implements.
Another option to get a similar ability would be to take the Shadow Initiate multiclass feat which allows you to use the Assassin's Shroud power twice an encounter.