Battlemaster 4, Swashbuckler 3, Hexblade 1 currently. 10, 16, 16, 10, 12, 14. I picked up Magic Initiate at BM 4 and obtained a familiar, prestidigitation, and Booming Blade. Our story had us entering some god level interactions and the GM offered the group a deal for more power (warlock) if we wanted to spread ourselves even thinner. It felt like my character at that point might take them up on it, so I started looking around at what was to offer. I was originally going for Swashbuckler 13 / Battlemaster 7, but if I add 2 levels of warlock I can pick up a couple spells and a couple invocations while sacrificing an extra attack and a maneuver by dropping to Battle master 5. So I guess this is where I am, The best patron would be Hexblade for obvious reasons. I like very versatile characters and so far this build is crazy versatile and has a great skill spread. If I go Hexblade, it opens up the Hexblade's curse, cantrips: Eldritch Blast and Minor Illusion, and I was thinking about Devil's Sight and Eldritch Sight because my character is a detective/P.I. Spell-wise going for Hex, Shield, and Comprehend Language. I don't know much about warlocks, so I was seeing if anyone had a few tricks I am not sure of. I was not planning on taking Agonizing Blast because my character is a sword and board fighter with +2 to CHR. Also, I might pick it up later with a feat when Multiple blasts would pay off more damage-wise.

There's a classic warlock trick that'd work great with your other classes, but you'll have to get to level 3 in that class.
Using Devil's Sight, you can cast Darkness right where you and your enemies are. Unless they also have that/have blindsight, you have advantage on any attack on them. For a fighter/rogue, that's huge.

Just want to point out that Warlocks do not have a monopoly on making deals for power. You can roleplay that just fine without changing your build.

Think about the opportunity cost here. One more fighter level will greatly increase your attack options and potential damage through Extra Attack (I get you have BB but it is often the lesser choice when you have both). One more rogue level gives you an ASI/feat. You can just as easily say that those powers came from whatever patron you're making a deal with and it doesn't require your build to go off the rails and get spread too thin.

You can certainly find ways to make use of a couple Warlock levels, but you're not getting the full power of the dip with +2 CHA that will never likely be increased.

I know about the darkness trick, but it doesn't fit my scheme of flash and flirt fighting. I was mainly looking at hexblade because my character is a halfling detective and devil's sight and eldritch sight would let him add more ways to detect things. I will still get extra attack at 5th, so that is in my build. I just have felt that fighter level 6 and 7 don't feed into my build very much. Losing an ASI, an extra maneuver and die per SR, plus Know Your Enemy seems like a fair trade all the benefits of 2 hexblade levels. We are also in a pretty RP heavy game, so the advantage of an extra attack in combat is a little less useful. I am getting my sneak attack every round unless there are more than one attacker while I am by myself, so there is always a ton of damage getting loaded onto my rapier.

I was just seeing if there was anything I was missing about the warlock: invocations, spells, interesting mixes, is hexblade the best option?. Right now I am on the path for this much interaction versatility.

Reaction: Riposte, Uncanny Dodge, Opportunity Attack, Shield

Bonus: Second Wind, Dash, Disengage, Hide, Aim, Elegant Maneuver, Hex, Hexblade's Curse

You’ve already got plenty of features competing for your bonus action, piling the bonus-intensive Hexblade onto it isn’t likely to help. Folks take Hexblade to simplify their MAD build down to a SAD charisma-based one, but your Charisma is low and likely to stay there. If you did somehow boost Cha, battlemaster DC is still Str or Dex based. 

Youre heading into T3 with T1 stats, more features aren’t gonna help that unless you’re min maxing for a specific stupid combat trick like darkness bubble shenanigans. Since you aren’t... you need feats, man, start pushing fighter and rogue levels to pull your Dex up to 20, THEN re examine if warlock invocations can help your play style. 

Going Hexblade at this point is pretty much just to get Hexblade's Curse... you don't really benefit from the other features of the class at this point, since you're already a solid frontline fighter. Going off of your concept of your character being more of a Detective who finds themselves caught up in a lot of RP Skullduggery... might I suggest Great Old One? It gives you at-will telepathic communication that bypasses language barriers... perfect for interrogations, sending secret messages to allies, and a dozen other things that being telepathic can pull off.

Spell-wise, it gives you access to Hideous Laughter, which is one of the best  first level battlefield control spells in the game and remains useful even into high-level play. If you choose to invest one more level of Warlock than your initial plan you could also learn Detect Thoughts, which has pretty obvious benefits for a Detective character.

I love, Love, LOVE the Great Old One's warlock path. I was really leaning into it pretty hard at first, but just figured since I was a fighter/rogue the Hexblade seemed to make more sense. It is funny you pointed that out, because I have been wrestling with my DM about which would be more interesting. When I chose my maneuvers, I selected Riposte, Menacing attack, and Studious Eye because it felt like they would be the most useful for my character. Riposte and Menacing Attack fits with the flourishing and intimidating fighting style of a classic demoralizing fencer and Studious Eye has been used more than anything else on my sheet.

Your right about the Hexblade's Curse being the only real advantage. I figured it was an acceptable trade for losing 2 HP, 1 maneuver dice, and 1 maneuver to get such a powerful ability replacing them. Add on top of it some great debuff and utility spells, uber-darkvision, and an at will detect magic or disguise self (still weighing those options). Those 2 fighter levels are comparable trash when you are multi-classing.

If I were to go GOO, I lose a superb combat ability and the shield spell for super telepathy, gain access to 2 WIS save spells with my DC being low because my CHR will never go above a 16. The spells are WAY BETTER for sure.

Shield on a Warlock is sort of a false resource: you have so few spell slots per fight that Shield is never really an attractive use of it.... and with Uncanny Dodge being an powerful Defensive use of your Reaction, and Riposte as a great Offensive use of your Reaction, I'm not sure that your combat decision tree will leave very many contexts open for Shield, like it would for an Eldritch Knight or melee Wizard that didn't have those competing abilities available. 

I was thinking of shield as a last ditch option if I was ever surrounded by multiple opponents or getting multi-attacked by something, and in a situation I was unable to get out of. A defense for every occasion. Riposte and Uncanny Dodge only work against a single opponent, but shield is an option for multiple attacks. At least that is my take for a spell everything says is a terrible choice. I know two spell slots can get chewed through fast in an encounter considering one WILL always be used on Hex.

My whole character is a short rest machine TBH, a ton of one shot abilities that refresh on a short rest. Versatility without over-specialization. DEX 18 with one more level of rogue and an AC of 19, maybe higher if I can find a +1 shield or armor. Already have a +1 rapier which can be used as a focus because I acquired a Ruby of the War Mage. I am interested to see how it plays out in a higher tier of play. I have played a few different characters to 15, but they were all single classes with maxed out primary stats. It felt like overkill to me most of the time. This is my first multi-class character and I am really enjoying feeling like a Swiss army knife.