It's still not clear on what you're trying to accomplish exactly.

Are you just trying to have a narrative reason for taking a warlock multiclass? Doesn't really seem like that falls outside of DM jurisdiction to me.

Are you trying to advance in character levels faster than usual? I don't think most DMs would do that, but I don't see how it falls outside of DM jurisdiction and I would think it's AL compliant as long as it's not something dumb like "I'm a level 4 bard/level 1 warlock multiclass with all the benefits but my total character level is still 4" or some wackiness like that.

Are you trying to make up some weird special snowflake benefits that would clearly go outside the bounds of the established rules? It doesn't sound like that's the goal but obviously that wouldn't fly in an organized setting like AL.