D&D 5E Green-Flame Blade Synergy?

[laughinggas]

Does the +CHA to damage from undying light warlock and fire dragon sorcerer stack on green flame blade?

Does it apply to both instances of the fire damage?

If divine favor was active, would that instigate another +CHA on the main attack?

so at lvl 7 with 20 DEX, 20 CHA a GFB with a rapier could do 1d8+5(DEX)+1d8+5(Elemental affinity)+5 (Radiant Soul) and to an adjacent target do 1d8+5(CHA)+5(Elmental Affinity)+5 (Radiant Soul)

 

[TwoSix]

While there's no clear rule, it's a general principle (via Jeremy Crawford's tweets) that class abilities DO stack. So a UL warlock 1/dragon sorc 6 would get Cha added twice to their fire spells, including greenflame blade.

I believe, though, that the second target of the GFB effect would not get the bonus damage, under the v1.1 errata that says only one damage roll can gain the benefit of the sorcerer's elemental affinity ability. You can make some arguments here (the warlock ability isn't actually called out by the errata, since it's playtest material. And before 5th level, the cantrip doesn't actually give a damage roll to the second target), but I think it's the better ruling.

As someone who's playing a fire dragon sorcerer with good melee stats that uses greenflame blade, I'll point out that the splash damage doesn't come up nearly as much as you might think, so granting the second target the bonus damage isn't wildly broken or anything.

 

[Blue]

From the Sage Advice Compendium:

Can you confirm that the bladesinger’s Intelligence bonus to AC is supposed to stack with other AC bonuses?
A bonus always gets added when the game instructs you to add it, unless you’re already receiving the bonus from a game feature with same name.

Since the Undying Light Warlock and Dragon Sorcerer features are different names, the answer is yes.