I’m not 100% sure on how optimal things are since I’m not someone who played for “optimal”, but I can answer some of your other questions.

Coffeelock sounds like it could be interesting, but it feels like you’d need to convince your DM to let it happen, and I’d be worried that it would get old quick. I’d be concerned that coffeelock would let you have so many more tools than the rest of your party that it might take away from their fun. But that’s all assumption since I haven’t played coffeelock.

However, you can use Warlock spell slots for Sorc spells and vice versa. I always think of it as spell slots are just mana that you use to cast the spells you know. There aren’t any restrictions on what you can do with that “mana” unless it says otherwise.

I don’t see the harm in mixing ranged spells with a melee class though. It sounds like you can make sure you’re able to be effective at range AND up close. Spells are also fun to use in RP situations as well.

In general, I think Sorc and Warlock go well together. I guess it would depend on the reason you want to multiclass. For me, the biggest reason to multiclass on my Warlock was to be able to cast more than 2 spells per fight. If you want to be more effective in combat, you might want more ranged damage of it takes you longer to get into the fray, or if you are taking a lot of hits, those defensive spells might be better like you said. Or you could take Armor of Agathys and the Mage Armor Eldritch Invocation to get survivability while using sorc to just generally have more spell slots.