Can the Animate Dead spell be cast on a zombie?

One of the players in my game is a Death Domain cleric and enjoys having a small army of zombies accompany him. We had come to a situation where it is not plausible for him to keep the zombies with him for the 24-hour period, but he spent the change (on a government writ) to get them locked up until his return, so he can try to regain control over his minions.

Is it possible to cast Animate Dead on a Zombie and have the spell work as normal? Is a zombie considered a corpse, technically?

Also, if you say that it should work in theory, here comes the sticky part. He also wants to use this spell on a Mummy Lord (that he doesn’t know is a Mummy Lord, yet). Does it simply not work, since it’s an undead creature and not a corpse?

Can a druid of spores revive a zombie as a reaction if the humanoid died last turn?

There is a class feature for spores druids called Fungal infestation, and in the rules it is stated:

If a beast or humanoid that is Small or Medium dies within 10 feet of you, you can use your reaction to animate it, causing it to stand up immediately with 1 hit point. The creature uses the zombie statistics. It remains animate for 1 hour, after which time it collapses and dies.

So, if a bandit dies within 10 feet of a druid of spores, and the druid doesn’t revive him (already spent reaction on the turn or whatever) can he use the reaction to revive next turn?

How many actions can a summoned zombie use?

Many zombies – such as the Zombie Shambler – are permanently slowed 1, which reduces the number of actions they can use by 1 each round. Summoning such a zombie with a spell (such as Animate Dead) gives the zombie the summoned trait. This makes the creature a minion with only 2 actions that it can use per round.

Since the zombie is permanently slowed 1 and starts with 2 actions, one would be led to believe that such a minion could only use 1 action per round. However the summoned trait has this clause that casts some doubt on that assumption:

Immediately when you finish Casting the Spell, the summoned creature uses its 2 actions for that turn.

Does this grant the newly-summoned zombie a second action to use? If so, is it for that turn only, or for the duration of the summoning?

Does a zombie or skeleton explicitly need to have spoken in life?

The SRD claims that both the Skeleton and the Zombie have the following mastery in languages:

Languages Understands the languages it spoke in life but can’t speak

I’ve always taken this as that the original creature that turned into the sekeleton or zombie, mastered a certain language and took this knowledge with them when they became an undead creature. However, when looking at the Specter, I noticed it said the following:

Languages Understands the languages it knew in life but can’t speak

Is this difference intentional? Does the skeleton or zombie explicitly need to have said something in the language it mastered, or this is just a small overlooked difference that I should ignore?

I suppose it’s a situation that rarely comes up, but it could be relevant when a mute creature becomes a zombie. Certain creatures can understand but not speak certain languages, so it could be relevant for those whether their zombified undead could understand such languages.

Does a zombie or skeleton explicitly need to have spoken in life, or is this an unintended difference?

Does a size large or larger creature killed by Negative Energy Flood rise as a medium sized zombie?

The spell Negative Energy Flood (XGtE pg. 163) states:

A target killed by this damage rises up as a zombie at the start of your next turn.

And also,

Statistics for the zombie are in the Monster Manual.

This seems to imply that the creature killed rises as a zombie with exactly the statistics stated in the Monster Manual.

If I kill a gargantuan (or anything larger than medium) creature with Negative Energy Flood, does it rise as a size medium zombie?