No

But the argument is not so straightforward.

The problem with this question is that the RAW are murky enough to allow for some leeway in interpretation. Going by RAI, the intention was clearly to use Two-Weapon Fighting with one-handed weapons, and not game the system by attacking with a versatile weapon while holding it in two hands, arguing that it still counts as a one-handed weapon for the Dual Wielder feat.

The Rules

Dual Wielder

You can use two-weapon fighting even when the one-handed melee weapons you are wielding aren’t light.

While this qualifies the longsword for two-weapon fighting, the Versatile property states that:

Versatile

This weapon can be used with one or two hands. A damage value in parentheses appears with the property—the damage when the weapon is used with two hands to make a melee attack.

This explicitly states the need to use both hands to make a two-handed attack and use the bigger weapon dice, thus disqualifying this use case from the Dual Wielder requirement (one-handed melee weapons).

The Conclusion

While you can use a thrown handaxe and then attack with the longsword, you can only do so using the one-handed dice the Versatile weapon tag provides.

You cannot use Two-Weapon Fighting with Gauntlets

Two-Weapon Fighting states (emphasis mine):

When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. [...]

The rules of fifth edition D&D default to natural language when a term is not precisely defined elsewhere in the rules. Thus the meaning of "hold" defaults to standard English. If somebody were wearing gloves or gauntlets and told me they were holding them, I would say that they were wrong. To showcase this the sentence "he is wearing gloves" gives a very different image than the sentence "he is holding gloves".

You do not gain the +1 bonus to AC

The Dual Wielder feat states (emphasis mine):

[...] You gain a +1 bonus to AC while you are wielding a separate melee weapon in each hand. [...]

Similarly I would not say that you are "wielding" gauntlets, instead, you are wearing them and using them to attack. However, I could see the argument made that you do wield gauntlets when wearing them (or using them to attack) but then you would be benefitting from one part of Dual Wielder (the +1 AC) and not another (the Two-Weapon Fighting) which is a bit of a disconnect for me and another reason I would say that worn gauntlets are not wielded.

Power Armor already has a lot going for it

Power Armor grants you an entire weapon (1d8 thunder damage is nothing to scoff at), it can be used as a spellcasting focus and it does not limit the use of your hands in any way whatsoever.

Compare this to the Claws of the Umber Hulk which grant you a 1d8 slashing damage weapon but also prevent you from manipulating objects and casting spells with somatic components, all while requiring attunement (granted, they also get you a 20 foot burrowing speed).

There is a sizeable difference in power between these two, not to mention all the other benefits of Power Armor from various infusions and its own inherent versatility. That doesn't necessarily mean Power Armor is too strong, but it's something to keep in mind when buffing the feature.

In the end, it's all playtest; feel free to try things out

Of course, all this is playtest material and so it isn't going to be as heavily edited, word-curated, or, well, playtested, as the official content. Feel free to try out various interpretations and ideas to determine how imbalancing (if at all) it would be to allow for various features to apply to Power Armor's gauntlets.