I’ve been trying to increase the AoE of the manifest armor skills whirlwind and sweeping melee attack (triggered by shield throw). Both of these abilities should be melee and area skills. But it seems neither ability is impacted by increased area on gloves nor increased area for melee skills on weapons. Is this intended? It makes the shield throw manifest armor in particular extremely weak as the AoE is pitifully small.
Try passive form FG tree and idols.
There aren’t any forge guard nodes that increase the area of effect of minion skills. I don’t think there are idols for that are there?
The idea here was that I put a t7 increased area for area skills on my gloves, then maxed out the +240% stat increases from gloves for Manifest Armor. This should have given Manifest Armor AoE abilities a huge increase in area, but it did nothing. Similarly, if you have unlocked the Manifest Armor node that gives it your weapon, if you have increased area for melee area skills on your weapon it will also not increase the area of the melee AoEs for Manifest Armor.
No, there’s idols that do area for Forge Strike. And there is a node at the end of FG that at the 5 pt threshold gives you extended area based off of how many stacks of stalwart you have on a 10+ mana cost skill. But I don’t recall anything for area and minions in FG.
Have you done testing to know for sure? Because it should get areal as the tags for the minion have area in the melee and whirlwind strike tags. https://www.lastepochtools.com/minions/manifested_armor
So you should probably file a bug report.
I think it is a bug. There is one node in last line of fg that should increase area but I asume that it is also bugged 
According to 5.19 Minions “Modifiers your character has (such as from passives and items) will not apply unless minions are specified.”
So in general, the affix would have to say “increased area for melee minions” or something like that. AFAIK no affix like that exists.
Manifest Armor is special, because it should gain stats from your armor (and from your weapon if you have the Titan Sword node), but I couldn’t find any information about which stats exactly are getting transferred. I mention this because this post Question about Manifest Armor stats taken from my gear - #13 by greenwater2642 says that not all stats are actually transferred.