Hello, I am a very new player to D&D on my first campaign. I chose a warlock with a fiend patron but at level 3 a problem developed: the patron was evil and the party was good but people, including myself, liked the idiot-pyromaniac character and I was reluctant to Mary-Sue the character personality. The DM and other players are highly experienced so they help advise when I miss something.

After much discussion, I agreed to multiclass into thief and a new patron with a good alignment was "found" to replace the evil one but I was left with a warlock-3/thief-1 following the warlock/fiend path here on D&D Beyond with a good-aligned patron, ban on necrotic/undead/evil anything, no evil minions and no thievery or murder.

Right now, the character is Warlock-Fiend-3 / Rogue-Scout-7. The campaign is supposed to run until level 20 or beyond. My plan has been to refocus on getting warlock to at least level 9 to maximize spell slot level, perhaps as high as 13.

Fighting style is mainly a dagger with sneak attack that can be thrown when I do not use eldritch blast or cast a spell. I originally hoped to develop the character for group utility and took pact of the tome. Cantrips and rituals are intended mostly for out-of-combat use.

D&D seems like it has a balance problem when it comes to multiclass but maybe I misunderstand. Taking one level of another class seems powerful but more than that appears to impede development so, for example a sixth level character with three levels in two professions works like two separate third-level characters the player must choose from instead of one sixth-level character.

I already figure on doing half the damage of the other players but does anyone have advice on being better at utility or damage as I take the character more toward half warlock and half rogue scout?

Quote from Anchoku >>

 

D&D seems like it has a balance problem when it comes to multiclass but maybe I misunderstand. Taking one level of another class seems powerful but more than that appears to impede development so, for example a sixth level character with three levels in two professions works like two separate third-level characters the player must choose from instead of one sixth-level character.

 Multiclass can be very powerful, but there are a lot of terrible combinations. I would never advise a very new players to multiclass and I think your experienced friends did you a disservice with their advice (as an aside, there's absolutely no reason you had to multiclass to switch patrons. You can handle that entirely through roleplaying).

However you could do a lot worse than a warlock/rogue combo. You should already be pretty good outside of combat with rogue's extra skills and warlock's utility spells, so I'd focus on stuff that helps you scout, explore, and deal with social situations. If you're concerned about damage, make sure you have the Eldritch Invocation Agonizing Blast. You won't be a superstar, but you should be able to contribute just fine since your Eldritch Blast scales independently of your warlock level. A split like 7/13 isn't great though - most combos only dip a couple of levels into their secondary class to get a specific key feature. I wouldn't have put that much into rogue unless I was going to be playing mainly as a rogue. If I were playing the character, I'd feel like casting a spell is wasting all the investment I put into Sneak Attack. I will leave the advice of where to go from here to those who have a bit more experience with the combo.

Agreed.   I have never felt that more than 5 levels is ever worth it for any multi-class.   A lot of reasons to go 1-3,   4 gets you an ASI/Feat.   The only reason to go 5th level is that most classes get a big boost at 5 (Third level spells for example).    Six or seven is almost always a bad decision.  Never seen it worth while.

Is it allowed to pick up a cantrip like Green Flame Blade, which adds 1d8 damage to the target at level 5 and 11, to the 4d6 for level 7 rogue? The 2d8 at warlock level 11 might help a little on the missing increases of sneak attack.

The character has a low intelligence and wisdom and is forbidden from using Necromancy-type spells, any illegal acts, or adding to the list of characters needing combat turns (summoning).

A level 4 warlock gets another cantrip, so you could do it that way. And cantrips, btw, scale with total character level, not class level, so at a 4/7 split, you’d get the level 11 version of it. Or you can always ask your DM if he’ll let you swap out one you already have for it. 
Like others have said, they seem to really have steered you in a weird direction. Did you know there were going to be these restrictions at character generation? You might really ask if you can just rebuild your character from the ground up. Keep the personality and all, just go either straight warlock with an approved patron, or go straight rogue. Multi classing incorrectly is really one of the only ways to make an ineffective character. Play the, I’m new and didn’t know what I was doing or understand the character restrictions card. See if you can just respec as a single class. 

In most scenarios, multiclassing half one class and half another is not recommended. It almost always makes you lose an ASI, and most important class features that are what you're multiclassing for are not granted at that high of a level. Smaller dips into different classes (from level 3-5) are normally more often to be recommended. 

The issue with the one team member having an evil patron did not arise until after several months of weekly play and was not brought up during character and backstory creation. After determining the Fiend had to go, options like converting to a monk, fighter, barbarian, sorcerer, wizard or even arcane trixter were discussed but the character had high charisma, medium strength and dexterity, average constitution, low wisdom and very low intelligence. It was a difficult discussion but I agreed to multiclass into rogue after three levels of warlock. Other requirements such as being "good" helped push the character toward the scout archetype and the dungeon master helped create a new patron and sort of a personal story arc that allowed the character's personality and backstory for continuity. A lot of work went into redirecting the character's development and realigning the character without making it look like the character was killed and replaced by a totally different one.

I agree the character was steered in a strange direction but still has the same personality (idiot and childish), interests (fire, food, rituals), strengths (charisma) and weaknesses (low intelligence, afraid of deep water, cannot swim, gets sea sick, hates cold). The character might be most similar to an arcane trixter but with more potent spells once warlock gets to 13 and has two spell slots at 5th level and a couple more invocations. Spell slots will be saved for area-of-effect attacks while most other attacks will be either eldritch blast or a finesse weapon like dagger. I just hope it will not turn out too deficient.

I’d say either have an out of character, one on one conversation with your DM and see if you can rebuild. Tell them you didn’t understand the restrictions. (Don’t accuse them or say things like, you didn’t tell me, that gets combative.) And ask if you can just rebuild your character from scratch now that you better know what you’re doing. 
If not, maybe just stick with rogue all the way through. It’s not an optimized multiclass, but it could still probably be a pretty fun one. A rogue with a few unusual tricks added in.