Mark_GGG

Do the Vaal double strike clones attack with the same support gems I'm using?

Yes. They are specifically using your Vaal Double Strike skill (but cannot create more doubles with it).

And are they snapshots of my current buffs etc. or are they affected by my buffs in real time?

They are using your skill. That skill updates as your stats change.

Technically yes. They are allies, so will probably receive the buff, but that doesn't matter because they don't have any skills of their own which could benefit from improving their damage.

They use your skill, so Rallying Cry giving you a more damage buff will improve the damage of that skill This bit was unclear and confusing because I used the wrong terms. If an ally uses Rallying Cry, that gives you an added damage buff, which will affect your skills as used by the warriors. Rallying Cry never gives a more damage buff, it gives more damage to exerted attacks, which only applies to attacks you perform - Exerting isn't about the skills being yours, it's about you being the one using them.

It's definitely a spell

No, it's an attack.

the wiki page for it says "You cannot Cast this Spell directly".

It does, but that's the wiki being weird. The skill in-game does not say that.

The buff (added damage equal to % of main-hand weapon damage) is only for allies. The bonus it applies to you is the more damage modifier for the attacks it exerts (which scales with number of allies).

Warcries are not spells, they are not affected by cast speed modifiers.

They can't really be particularly syncronised in the first place, since they're tracking entirely different things. Glacial Hammer is tracking successive strikes with the skill, and a given skill use will make one or more strikes (depending on when it repeats). And they have to be directly successive, doing anything else "breaks the streak".

If you never repeat with glacial hammer, so each skill use does a single strike, then Ruthless is still going to be every third use, while glacial hammer's bonus is applied if you use the skill (and thus strike) three times in a row without doing anything else, so they'll be doing different things if you ever use the skill a number of succesive times that's not a multiple of 3.

What do you mean by "desyncronised"? That doesn't sound like a thing that can happen.

Ruthless counts uses of a skill, and applies to every third use. Multistrike causes each use of a skill to repeat twice. That doesn't cause more uses. If you have both on the same skill, then every use of the skill will have two repeats, and every third use (including its repeats) will be Ruthless and deal more damage.

Channelling spells fundamentally cannot be triggered by any means.

Yes. "Modifiers to damage taken" means modifiers to the damage taken value, after it has been calculated (via resists/armour). Modifiers to which type of damage you take happen before any mitigation because otherwise there would be no point.

Would that mean the 70% Elemental Resistance applies first and the reduction from Armour is applied afterwards?

Yes.

Meaning it adds the 70% first and then the % from Armour as total (let's say 10%), e.g. 80%?

No. That would be applying them at the same time.

Armour's damage reduction is calculated after Resistances are applied, e.g. if you have 70% Resistances you only need to account for the remaining 30% for the Armour damage reduction calculation

This is correct.

"[type] Resistance" applies before "[type] Damage Reduction" for any damage type that you have both those mitigation mechanics for.

As always, both apply before any modifiers to damage taken.

Righteous Fire is a buff applied to the player, it doesn't use any origin location, so Arcantist Brand is not any different from using it with any other trigger.

Arcanist brand Triggers spells.

Maybe a stupid question, but will damage be reflected to players if spells hit triggered by Arcanist Brand?

Yes. Just like any other time your spell is triggered.

No.

So Fireball + Arcanist brand + LMP without pierce will hit the branded enemy and then no one else

Projectiles don't shotgun, so with the LMP in there, one of the fireballs will hit and explode on the branded enemy, the other two will be unable to collide with that enemy and will continue through to hit other things.

Pierce or chain are definitely good things to have for projectiles used this way, though.

It will be compatible, but will not behave any different to any other method of triggering blade vortex.

There's no useful interaction there - they'd cast the spell, gain the blade, immediately die, lose the blade. It's the same reason they can't use Righteous Fire or auras. Any skill that applies an ongong thing to the caster doesn't work if the caster ceases to exist as a result of casting. Because it wouldn't work, making the support not apply is the best option, because otherwise it would be confusing to players when it supported the skill and looked like it never used it.

There is a theoretical case where it could actually do something, but unfortunately that's horribly abusable and thus can't be allowed - if you have the stats to allow your mines/traps to re-arm to detonate/be triggered an additional time, it would gain one blade, and then sit there not being targetable but dealing damage with the blade, until it dies after the second detonation.

That can't be allowed a) because we can't change supportability based on that modifier, and b) because it's a...

Being triggered in a certain way does not change what type of skill something is. Nothing becomes a Brand skill due to being triggered this way, so modifiers which specifically apply to Brand Skills will apply to the Arcanist Brand itself (if applicable), but not to non-brand skills it triggers.

However modifiers specifically to Damge, Critical Strike Chance and Critical Strike Multiplier of Brand Skills, have all been changed to instead affect "Brand Damage" or "Brand Critical Strikes", which are new keywords defined as Damage/Critical Strike Chance of Brand Skills or any skills triggered by Brands. There will be reminder text on sources of those stats explaining that definition.

So for the damage and crit chance/multipler stats specifically, those have been changed to something which can apply to spells triggered by Arcanist Brand. Any other modifiers which still refer to Brand Skills will continue to only affect Brand Skills.

Brands are extremely dissimilar to totems, it's certainly not safe to assume any mechanical things will work the same way between the two.