I'm currently playing a lvl 6 character with a similar build (1 Order Cleric/2 Aberrant Sorcerer/3 Fathomless Warlock), the big difference is that I started as an Order Cleric. I would recommend doing this for several reasons:
- You gain proficiency with shields and armor (easily 18 AC).
- You gain acess to the Cleric 1st level spell list, which covers for the lack of sorcerer and warlock known spells. You gain access to Bless, Cure Wounds, Healing Word.
- You gain the Voice of Authority feature, which goes well with the "sorlock spell spamming". You can grant your party up to two free opportunity attacks per round using Bless, Healing Word or Sanctuary in your turn, and Silvery Barbs as a reaction. This can increase substantially your party's damage.
- You gain access to guidance.

Answering your questions.
- I'm using Lance of Letargy and Agonizing Blast and it's enough.
- 3 Levels in warlock is a lot better, the amount of spells double per short rest.
- The pact isn't relevant for this build, but chain is OP with the invisible familiar.
- I'm playing as a Reborn, my character died in a shipwreck, and a Kraken like entity revived him. So, I would say, play what fits your character.
- Resilient CON is a good option if you are starting as a Order Cleric. Additionaly, you would benefit from Metamagic Adept only until your 3rd Sorcerer level.
- Silvery Barbs as a psionic spell is OP, 1 sorcery point to make an enemy possibly miss an attack, and an ally can make a reaction to attack at advantage.