Still new to the community, so my apologies if my formatting isn't perfect or my questions are unclear. I'm in the process of preparing my first One-Shot as a way of trying DM-ing in a few weeks.

For one of the battles, I was thinking of having the PCs face an Animated Armor and a Flying Sword. According to the Dungeon Master's Basic Rules (page 9 and 20 respectively) both of these enemies...

If targeted by dispel magic...must succeed on a Constitution saving throw against the caster's spell save DC or fall unconscious for 1 minute.

After reading this, I thought it might be cool to hide a Scroll of Dispel Magic earlier in the dungeon, which could help in this fight. But having thought about it I came upon some possible issues of confusion.

What is the effect of an enemy being temporarily unconscious during combat?

According to the PHB page 234, the target of Dispel Magic is...

One creature, object, or magical effect within range.

So at most the Scroll of Dispel Magic would affect one of the monsters. Say a PC used the scroll on the Animated Armor and it was knocked unconscious for 1 minute (approx. 10 rounds of combat). Does the Animated Armor now act like a dead enemy (i.e. does it become non-targetable) or does it essentially just freeze for 10 rounds of combat while the PCs can attack it mercilessly?

What if the other enemy is defeated?

Same scenario as above, say the Animated Armor is hit with Dispel Magic and becomes unconscious. The PCs now focus on the Flying Sword. Upon killing the Flying Sword, all enemies are either defeated or unconscious, so would the combat now be done and the Armor would re-animate after a minute or less of non-combat and combat starts again? Or would combat continue until the unconscious Animated Armor is defeated as well?

Would including a Scroll of Dispel Magic break this fight?

Essentially I ask all these questions to see if including the Scroll would make this fight too easy/useless. The plan was for this to be the boss battle or near to it, so I was thinking that it could be nice to add something that could make the battle a little bit easier, but I don't want something that is going to turn the fight into a total joke.

Hopefully that all makes sense! Please feel free to modify or ask for clarification if I have done something wrong or am unclear!

Unconsciousness is a status condition in 5e, and has mechanical effects

From the Player's Handbook, Appendix A "Conditions", Unconsciousness is described like this:

Unconscious

• An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings.


• The creature drops whatever it's holding and falls prone.


• The creature automatically fails Strength and Dexterity saving throws.


• Attack rolls against the creature have advantage.


• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

Given these effects, we can evaluate your assumptions and questions:

Does the Animated Armor now act like a dead enemy (i.e. does it become non-targetable) or does it essentially just freeze for 10 rounds of combat while the PCs can attack it mercilessly?

Players can indeed attack attack the armor mercilessly for the duration. Advantage + nearly-guaranteed critical hit damage will render the armor or sword dead very quickly.

What if the other enemy is defeated?

If the players don't actively kill off the armor when it goes unconscious from the Dispel Magic effect, then it will "wake up" in 1 minute, and begin attacking again.

You may choose to maintain combat initiative order during that time, or you can dismiss combat and call for initiative again after that minute passes (which may psych your players into a sense of false safety). There's a non-zero chance your PCs might leave the room before it wakes up, and leave it behind, still "alive"! You'll need to make a call as DM to determine what the right call is here, which you'll learn and get better at as you spend more time DMing or playing with good DMs.

Would including a Scroll of Dispel Magic break this fight?

Probably.

That might be okay, though. You don't need to tell the players that Dispel Magic has these effects (they might be entitled to that information on a good Arcana check, if they suggest it), and if the players manage to acquire that scroll, it might be a cool bonus to use it to skip a fight... or save it for a future encounter, depending on how they choose.

Bear in mind that a Scroll of Dispel Magic may only be used by a spellcaster that has Dispel Magic in their class list (i.e. no Rangers), and if they're not high enough level to cast the spell (5 for Sorcerers/Wizards/Warlocks/Druids/Bards/Clerics, 9 for Paladins, 13 for Eldritch Knights/Arcane Tricksters/Artificers), they'll have to pass a successful spellcasting check just to use the scroll. And since the armor also has a chance to resist the effect with a successful Constitution Saving Throw, having this scroll accessible to them is absolutely not a guarantee that the players will be able to instantly cheese past this fight.

The armor is treated as unconscious which means

• An unconscious creature is incapacitated (see the condition), can't move or speak, and is unaware of its surroundings.


• The creature drops whatever it's holding and falls prone.


• The creature automatically fails Strength and Dexterity saving throws.


• 
  Attack rolls against the creature have advantage.


• Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.

One minute is ten rounds of combat in 5e, not six. If the armor or sword is dispelled, it's as good as toast.

Since whichever enemy that wasn't dispelled would still be fighting, you'd maintain initiative until the second one was destroyed. Then, the party could circle up around the first one and bash it into oblivion to wrap up the fight.