Z. H. Darkstar
Explorer
You're right about taking Fighter 1 at the start for heavy armor proficiency. Combine that with Mobile, and you can dump Str while maintaining 30' speed in heavy armor. See my build above for how this comes together nicely.Would it not be better to go fighter first to gain access to heavy armor? That's AC 20 with the shield. Fire dragon's Element Affinity works on only one damage roll per spell. GFB has two seperate rolls, so you might have to choose, which one gets affected. Also as of errata you might not be able to twin GFB.
You're almost always going to add EA to the primary target damage of GFB, as there's no guarantee of a secondary target to be present each casting.
As for Twinned Spell, GFB isn't compatible as far as I can tell. However, it is still capable of working with Quickened Spell without the mandatory targeting of a different creature of TS. It is a cantrip after all, so it can be cast twice in one turn via QS. At the level it would all come together, you're looking at 4d8+16 to the primary target and 2d8+8 to the shmuck stupid enough to be next to the guy getting whacked with a flaming stick. One more level adds another 2d8 to each set of potential damage dice for the QS GFB nova rounds. Stack Hex on top for an extra 1d6 per hit on the primary target. This is how that would ideally play out.
Turn 1: Drop Hex on target, Eldritch Blast, move closer to the target
Turn 2: Close the gap, Shillelagh, GFB
Turn 3: GFB Nova!
After the first guy dies, move Hex to the next victim and GFB/EB depending on range. Resume Nova once in range. Use your sorcerer slots to cast Hex, so your warlock slots can feed your metamagic habit.