[RPG] the earliest the warlock can gain access to the Nondetection spell, preferably without multiclassing

Check out character themes, namely the Guttersnipe^DDI (featured in Dragon Magazine #399). Should provide some nice flavor as well as a few mechanical bonuses.

This should give your warlock flavor without making any sacrifices and still get the rogue elements you might like.

Your problem isn't that you need to persuade your GM to change how the Warlock class works. Your problem is that you've fallen prey to what I'm going to start calling "the Nomenclature Bugbear."

Each character class is a collection of abilities built around a concept frequently found in works of fantasy fiction. These classes are given names, presumably because it gets awkward saying "I'm playing a person-who-uses-weapons-and-armour-really-competently!" all the time. For example, the fighter class can use armour and weapons well, and the wizard class uses magic and keeps spells in books. You'll have worked this out in a few minutes of reading the class descriptions.

However, being a member of a class doesn't mean your character identifies herself with the name of that class. A Fighter could be a Viking raider, a knight errant, a conquistador, or any of a thousand other possibilities. Similarly, there are loads of characters in fiction who cast spells and keep a library, and most of them aren't called "wizard." The Nomenclature Bugbear arises when we forget that class names are a convenience of game jargon and start thinking that they're recognised in-universe.

To be fair, this is an easy trap to fall into, as often the words used for class names do exist in-universe: They were taken from the fiction that inspires the game, and most campaign settings are also inspired by that same fiction. Most D&D settings do have people called wizards, and most of them are members of the wizard class, because the wizard class is pretty good at representing what wizards are in that setting. You just need to remember that in many such settings, in-universe nomenclature doesn't necessarily line up perfectly with the game terminology: In most settings, you can introduce yourself as a "thief" and people won't assume your character is a Rogue with the Thief archetype, because the only in-setting qualification to be a thief is to steal things, and any class can do that.

Getting back to your example...

You want to make a character who's a witch in the campaign universe, a person who uses subversive magic as an alternative to physical might and social prowess. You want this to be supported by the game mechanics and your GM. Those are reasonable goals. To achieve them, you need to do two things: (1) You want to find a class that fits with your concept, and (2) you want to work with your GM so that you have a shared understanding of what your witch is intended to be.

As your GM has pointed out, the warlock class isn't really what you're looking for. But your concept is still pretty broad, and that means it's flexible. With your GM's permission, you can simply pick any class that fits it, perhaps wizard, cleric, or - well, anything that uses magic - and have your character call herself a "witch" in conversation.

Once you've picked a class whose list of abilities sounds like your vision of a witch, explain your idea to your GM. It sounds like your GM doesn't object to your character's concept, so I expect he'll be receptive. As long as he agrees that there's no major dissonance between what your character can do and what she calls herself, you'll find that this solves your problem. It might take a bit of back-and-forth if he has existing nomenclature plans for magic-users in the setting, but eventually you'll have a character that you're happy with, and which your GM understands well. It's win-win!