How does a Mastermind rogue's Master of Tactics feature work when trying to Help an allied spellcaster in combat?

Question

The Mastermind rogue's Master of Tactics feature (SCAG, p. 135; XGtE, p. 46) states:

Starting at 3rd level, you can use the Help action as a bonus action. Additionally, when you use the Help action to aid an ally in attacking a creature, the target of that attack can be within 30 feet of you, rather than within 5 feet of you, if the target can see or hear you.

How does a Mastermind rogue's Master of Tactics feature work when trying to Help an allied spellcaster in combat?

I think I have the gist of it, but need clarification on a few points:

• If a Mastermind rogue uses his or her Master of Tactics feature to Help a spellcaster in the party, would it grant advantage on a spell attack?
• I guess this would only work if the other party member casts a spell that requires a spell attack against a creature, rather than a spell that requires the creature to make a saving throw, correct?
• With regards to distance, does the Mastermind rogue have to be within 30 feet of the party member casting the spell? Or within 30 feet of the creature the caster is targeting?
• With regards to initiative, would the Rogue's turn need to be before the spellcaster's exclusively in the same round? Or, if the Rogue's turn came after the spellcaster, would Master of Tactics carry onto the following round, so that the spellcaster gains advantage in the next round instead?

Answer 1

If a Rogue uses his or her Master of Tactics feature to Help a spell-caster in the party, would it grant advantage on a spell attack?

Yes. A spell attack is an attack.

I guess this would only work if the other party member casts a spell that requires a spell attack against a creature, rather than the a spell that requires the creature to make a saving throw?

Yes.

With regards to the distance, does the Rogue have to be within 30 feet of the party member casting the spell, or within 30 feet of the creature the caster is targeting?

You must be within 5 feet of the target (basic help action) or if the target can see or hear you, within 30 feet (Master of Tactics). Your ally can be anywhere.

With regards to initiative, would the Rogue's turn need to be before the spell-caster's exclusively in the same round? Or, if the Rogue's turn came after the spell-caster, would Master of Tactics carry onto the following round, so that the spell-caster gains advantage in the next round instead?

From the Help action:

If your ally attacks the target before your next turn, ...

Your ally will always have a turn sometime between your current and your next turn.