Spells or magical abilities that allow multiple simultaneous effects?

Combining Magical Effects on page 205 of the Player’s Handbook says:

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

Are there any spells or magical abilities that allow multiple instances to target or affect a single creature simultaneously?

An example of such an effect is this or this.

Are special abilities with listed costs multiplied by 1.5?

My understanding is that you can basically “combine” magic items, but the second one costs 1.5x as much. So a ring of invisibility at 20,000 gp can be modified to also act like a +1 ring of protection for an additional 3000 gp. Alternatively, a +1 ring of protection at 2000 gp can be modified to also give invisibility for an additional 30,000 gp.

But what about the effects that have specified prices? If you have a plain old masterwork full plate armor, would adding acid resistance cost the listed price of 18,000 gp, or, because armor is a body slot, an inflated 27,000 gp? What if you add that, and then want to add electricity resistance? Since the armor already has an ability, is this considered an additional ability and thus costs 27,000 gp? Would the answer change if it were instead a +2 full plate or a +2 full plate of invulnerability?

Do racial/feat/non-Spellcasting classes’ spellcasting abilities count as a spellcasting ability for Staff of Power?

DMG p. 141 states the following (bolded for emphasis):

A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don’t have a spellcasting ability-perhaps you’re a rogue with the Use Magic Device feature- your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.

Staff of Power states the following (bolded for emphasis):

While holding this staff, you can use an action to expend 1 or more of its charges to cast one of the following spells from it, using your spell save DC and spell attack bonus: . . .

If you are a Tiefling Wizard, a Githzerai Sorcerer, a class who took the Magic Initiate or Spell Sniper feats, an Ancestral Guardian Barbarian, a Monster Hunter Fighter, or have any feature which grants you a spellcasting ability outside of the Spellcasting class feature, can you use the spellcasting abilities granted by these races/feats/classes when selecting your spell save DC and spell attack bonus for a Staff of Power (and other similar items such as Staff of the Woodlands, Voyager Staff, etc.)?

Tiefling (Infernal Legacy):

Charisma is your spellcasting ability for these spells.

Wizard (Spellcasting – Spellcasting Ability):

Intelligence is your spellcasting ability for your wizard spells, [. . .] You use your Intelligence whenever a spell refers to your spellcasting ability.

Githzerai (Githzerai Psionics):

Wisdom is your spellcasting ability for these spells.

Magic Initiate:

Your spellcasting ability for these spells depends on the class you chose: Charisma for bard, sorcerer, or warlock; Wisdom for cleric or druid; or Intelligence for wizard.

Spell Sniper:

Your spellcasting ability for this cantrip depends on the spell list you chose from: Charisma for bard, sorcerer, or warlock; Wisdom for cleric or druid; or Intelligence for wizard.

Ancestral Guardian Barbarian (Consult the Spirits):

Wisdom is your spellcasting ability for these spells.

Monster Hunter Fighter (Hunter’s Mysticism):

Wisdom is your spellcasting ability for these spells.

It appears to me that they all use the same wording so, RAW, this should work since Staff of Power lets you select any spellcasting ability that you have if you have multiple, correct?

How can a DM deal with abilities that affect how an attack roll is resolved?

Consider the following:

An Orc takes the attack action against a PC with an AC of 18. The DM rolls in front of the screen and gets a 14. He silently adds the +5 bonus from the Orc and announces that the attack hits.

Pretty basic. Now consider the same situation, but with a Lore Bard in the party:

An Orc takes the attack action against a PC with an AC of 18. The DM rolls in front of the screen and gets a 14. The Lore Bard sees the 14 and, while not sure of the exact attack bonus for the Orc, assumes that is should hit his companion. He therefore uses Cutting Words and in the end, the attack misses.

Okay, so here the DM had to wait for the bard to decide before announcing if the attack hit, because “you can choose to use this feature after the creature makes its roll, but before the GM determines whether the Attack roll or ability check succeeds or fails”. Simply having a Lore Bard in the game changes how the DM does his rolls.

But what if the attacked PC has the Shield spell prepared? Shield takes “1 reaction, which you take when you are hit by an attack”. Usually the DM will roll behind the screen and simply announce if the attack hits or not, letting the PC choose to Shield or not. Telling him the die roll would be meta information about whether or not the Shield will be wisely used or not.

So having a PC with Shield in the party forces the DM to roll behind the screen. But the Lore Bard needs to know the number on the die in order to decide on it’s Cutting Words! And if the DM asks the PC if he wants to use Shield because the attack hits, then it’s too late for the Lore Bard to use Cutting Words!

It seems to me that the DM must be aware at all time of the PC’s reactions that must be done in a specific window, to ensure that he presents the information in the right order and that he allows enough time for the player to decide.

How can a DM deal with this?

Bonus points for giving particularly problematic abilities to watch for when resolving an attack roll.

I am not interested in a list of all features that grant advantage/disadvantage or a bonus to a roll since most of those are resolved prior to the attack and are not step sensitive. For example, if the DM forgets to remove a d4 from a ‘Bane’d creature, the reduction can still be applied afterward.

Shield and Cutting Words are particular because they must be used between some steps and are affected by yet unknown information (actual die roll and attack bonus, respectively).

How do the Mizzium Apparatus abilities interact with each other?

So I recently got to DM my first campaign, and I was planning on letting my players get their hands on a Mizzium Apparatus. However, reading what the Guildmaster’s Guide to Ravnica says about the item, there is one main point I am confused about. For those that don’t know, it states that the Mizzium Apparatus can be used as an arcane focus while being worn, but it also has the ability to let the wielder cast any spell without knowing or preparing it, as long as the spell is from their class’s spell list, they have the appropriate level spell slot, and they provide the required components. I know that this means that they still can’t cast spells with verbal or somatic components if silenced or bound, but how does this work for material components? An arcane focus is used to replace the material components of a spell, assuming it doesn’t have a listed cost and isn’t consumed. So would the Mizzium Apparatus still work as an arcane focus for these unprepared spells, meaning they don’t need to pull out an arcane focus since its equipped, or would it only act as an arcane focus for their prepared spells? Or is it up to DM’s discretion with no set rules?

As a bonus, how would it work if they fail the arcana check and roll a spell from the list that they can’t provide components for (eg. they try to cast Catapult, fail the arcana check, and wind up casting Chaos Bolt instead despite being silenced)? Needing to be attuned by a warlock, sorcerer, or wizard, would a player multiclassing as another class be able to use it to cast a spell from that other class, such as a multiclassing sorcerer-paladin being able to choose from the paladin list despite paladins not otherwise being able to attune with it?