your right I wasn't clear, imo most people under value Armor of Agathys (maybe because they can't spell it). your analysis of it is pretty spot on. The idea is pretty much if you spam it when attacked, then you are doing lots of damage and mitigating a decent amount of damage that the party could be taking. Can it be over powered by many or powerful melee opponents yes of course, but can a fighter take be taken down in melee as well yes. The fact that you can heal yourself is also good since you may be the one needing healing. A good turn when focused on would be recast AoA, and heal yourself back up

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Do many of the abilities lend to a ranged style yes, but they are also great for melee. For example the 60 ft range on heal doesn't hinder you being in melee, in fact it means you don't need to move as much to use it and many times melee characters can't or don't want to move that much. Also the bonus fire damage at lvl 6 can be used quite nicely with Green Flame Blade (might not be a desirable after lvl 6, but debateable). Another thought I haven't ran the numbers but there is a good chance that Green Flame Blade combined with the lvl 6 add Cha damage to fire attacks might come out stronger than a second regular attack and allow you to pick something other than pact of the blade, Pact of the Tome gives you Shillelah meaning you could boost Cha faster at the expense of AC, dex save and init.
some numbers
Green Flame Blade lvl 6 with Shillelagh and boosted Cha lvl 4. 2d8 + 8 damage and 1d8+4 to another close enemy.
Much of what I've states has already been states on this thread (guess it pays to actually read stuff lol)
The celestial pact warlock seems like so much fun btw giving me so many character idea, in many ways its what I had envisioned for the favored soul.