Does the reach increase of the Eldritch Claw tattoo stack with Fangs of the Fire Snake?

The Eldritch Claw tattoo released in Tasha’s Cauldron of Everything includes the Eldritch Maul ability which grants the following benefit:

For the duration, each of your melee attacks with a weapon or an unarmed strike can reach a target up to 15 feet away from you, as inky tendrils launch toward the target.

The Fangs of the Fire Snake elemental discipline also grants the following benefit:

Your reach with your unarmed strikes increases by 10 feet for that action, as well as the rest of the turn.

After activating Eldritch Maul and using Fangs of the Fire Snake what is the reach of my unarmed strikes? Would it be 25 feet or would it be 15 ft?

Does Mind Sliver Stack with Bane?

The cantrip Mind Sliver from Tasha’s has a secondary effect applying negative 1D4 to a saving throw.

…subtract 1d4 from the next saving throw it makes before the end of your next turn.

The spell Bane from the Player’s Handbook has a similar effect.

…the target must roll a d4 and subtract the number rolled from the attack roll or saving throw.

Does the effect from Mind Sliver stack with the saving throw effect from Bane making it negative 2D4 to the next saving throw?

Do identical bonuses from random properties of different Artifacts stack?

Some Artifacts have Major/Minor Beneficial/Detrimental Properties, rolled from the appropriate table.

If I were to have two attuned Artifacts, both with, say, the Minor Beneficial Property of "While attuned to the artifact, you gain a +1 bonus to Armor Class.", would I have a +2 or +1 AC bonus?

From the DMG Errata on Combining Game Effects (p. 252):

Different game features can affect a target at the same time. But when two or more game features have the same name, only the effects of one of them—the most potent one—apply while the durations of the effects overlap. […] Game features include spells, class features, feats, racial traits, monster abilities, and magic items.

"Artifact property" isn’t listed, which is evidence for +2. But Artifacts are magic items, which may mean it should only be +1 instead. Which is correct?

Does damage reduction from Shadowfell Brand Tattoo stack with resistance?

Shadowfell Brand Tatoo, the new magic item from Tasha has this effect:

Shadowy Defense. When you take damage, you can use your reaction to become insubstantial for a moment, halving the damage you take. Then the reaction can’t be used again until the next sunset.

Does this reduction stack with reduction from having resistance?

Do Telepathic Talent and an elan’s resistance stack?

Do racial traits bonus stack?

Telepathic Talent:

Benefit: You can expend your psionic focus to gain a +2 trait bonus to your saves against mind-affecting powers and psi-like abilities. This talent grants no benefit if you do not have the ability to gain psionic focus.

and

Elan

Resistance (Su): Elans can use psionic energy to increase their resistance to various forms of attack. As an immediate action, an elan can spend 1 power point to gain a +4 racial bonus on saving throws until the beginning of her next action.

if I’m a elan (with focus), if is a mind-affecting power to resist, Can I use both?

Can the frightened condition stack if it’s caused by the same source?

Say Faendal the Fallen Aasimar Paladin uses his Necrotic Shroud racial ability (Charisma saving throw, an affected creature can repeat the saving throw on subsequent turns to end the effect), and a target fails the saving throw and is frightened. On his next turn, while the target is still frightened, Faendal uses Wrathful Smite, which also causes the frightened condition, on the same target and they fail the saving throw (Wisdom saving throw, an affected creature can make a Wisdom check to end the effect).

Does this mean that the target must succed on both the Charisma saving throw and the Wisdom check to end the effect on their turn? Meaning that should one of them fail, the target is still frightened?

Do multiple instances of the Extra Attack feature stack?

The WotC article Unearthed Arcana: Modifying Classes creates the "Favored Soul" sorcerous origin, which grants the the sorcerer the Extra Attack feature at level 6.

Say I made Favored Soul sorcerer go up to level 6, and then multiclassed into Ranger to get its Extra Attack at level 5. Would the two Extra Attack features stack so that I could get 3 or 4 attacks in a turn?