I'm more interested in if the basic idea works and I haven't missed anything obvious.
Okay, you know what? Lets do that. Ignore the stuff about single class paladin, and lets start fresh.
True Strike builds are often quoted as being frustrating. Until you get that paladin 3 level at character 9, you have a good chance of missing, and lots of people have said that they find it quite annoying. On average, that means just under half your turns, you're not doing any damage.
Rogues also only get one attack, but they have a ton of ways to generate Advantage. However, you do not have the same benefit by default - there's a few innate ways of giving you Advantage to raise your chances of landing a hit, but all with problems; figuring out what's best for you is important. Darkness/Devil's Sight combo might annoy teammmates. You can get Topple, but that's not going to benefit you, only your teammates. Your team might have topple, but there's a good chance that the enemy stands up before your turn. Vex is an option...assuming you hit first, and if it survives multiple rounds. Or use Lucky to start off. You could use familiar Help spam, but do note that familiars are made of fragilium and you want to be in melee~ Which brings me to the next point -
Warlocks are also notorious for their survival issues, and you're talking about being melee-primary and only taking paladin 1 at level 7. Warlocks tend to need to use their magic to boost their survival rates in melee - though Celestial warlocks tend to offset this with their Healing Light ability (which also takes a bonus action). Figuring out how you want to use your magic to aid survivability is a very important consideration at this point. Armor of Agathys tends to last longer under a Fiend (thp generation) or Fey warlock (misty step away), but less so for Celestial, who tend to rely on heal-tanking-to-the-face. So, I suggest putting more thought into survival stuff.
Or... If you're going to dip paladin, do you want it earlier so that you have heavy armor for longer time periods and delay that +CHA damage boost? Or are you going to rely on mage armor the entire time (which takes up invocation)? Don't forget that you're going to want 13 STR to wear heavy armor once you get paladin, but before that you want a good DEX score for the mage armor, and you're gonna want some CON for the extra hp. This is a lot of MADness, assuming you're gonna dump INT and WIS.
Which brings us to Concentration spells. You're going to want to avoid them like the plague. You've mentioned Hex before, which leads me to believe you haven't really thought of this aspect. You're a melee unit without CON save proficency nor enough feat slots to invest in War Caster. Your concentration check is laughably bad, and you're going to be making multiple per round. Do Not The Concentration.
Now, lets put a pin in that stuff and pivot to Invocations. Warlocks tend to rely on Invocations for lots of stuff, and you're very invocation heavy. Agonizing Blast, Eldritch Smite, at least. You need Tome if you do decide to use Shillelaugh, and possibly another for access to Mage Armor until you get to Paladin 1.
Note that, for your planned progression, you're not going to get any ability to smite until level 5. So your build is going to come online quite late unless you want to dip paladin early. Armor of Shadows or using Magic Initiate: Wizard to get mage armor is generally highly recommended, so that you have early level defenses - or use Fiendish Vigor. You'll eventually swap it out, but its critical for early stage survival. Honestly, its probably best to go Paladin 1 first, just so you have both higher AC, less need to worry about DEX on your build, and have access to smites early on.
You mentioned Shillelaugh, but clubs have crappy Mastery, and there's no reason to go quarterstaff with the spell, you've got a shield. You are better off going either Rapier (Vex mastery) or a Heavy weapon. Oh! Speaking of heavy weapons... True Strike builds work great with Savage Attacker origin feat. Its only once a turn, but... you're only attacking once a turn, meaning its a slightly better Fighting Style for Greataxes or other d12 weapons. Its less effective on a rapier/shield combo, but does stack with FS: Dueling.
Back to invocations - I feel like you're misunderstanding how Tome works a bit, because you want to stack it with a bunch of Magic Initiates with. This is ... kind of a waste. Just like asking to use Hex in combat. Tome doesn't give you acess to 3 cantrips and 2 rituals. It gives you access to EVERY cantrip and EVERY level 1 ritual, just a limited number at a time. If you need a niche cantrip, most of the time you'll be able to take a short rest and change them up. There's no real pressing need to stack Tome with more Magic Initates.
Yes, Magic Initiate also gives a level 1 spell. But you know what? You get invocations that give you spammable, free level 1 spells. This is why Otherworldly Leap is so excellent. Its free. Its spammable. So its effectively always on unless dispelled. This means that your combat speed might as well always be 50. You can jump over pits or walls without wasting a limited resource.
I'd also normally be singing praises for Dception when combined with Minor Image/Silent Image spam, or Disguise Self/Friend spam, but I feel like those would directly violate two of the Oath of Devotion tenents. Warlocks as a whole are very good tricksters in social situations, but Oath of Devotion is very anti-trickster. Check with your DM if you wanna invest in good social stuff.
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And that's everything I can think of off the top of my head.
EDIT - oh, yes, you're extremely susceptible to grapples, so make sure to keep a Misty Step in your back pocket in case of enemy shinanigans.