Hi beginning player here trying to learn the game

I have a human drunken monk 5 / fighter 1

An ally cast jump on the monk

Monk is melee fighting enemy A that's standing next to enemy B

At the beginning of the round. I have the monk jump up 30 feet and land on enemy A (spend 10 ft movement)

The jump does 3d6 blugeoning damage on enemy if enemy fails save and is knocked prone, and the monk takes nothing

Monk does 2 unarmed strikes and walks back, triggering opportunity attacks from enemy A and B

Let's say opportunity attacks both hit, but does only 10 damage each

Monk deflect attack both and reduce it to zero, and redirect opportunity attacks to A<->B

If enemies saild DC 12 dex save, they would take 2d8+4

is everything I'm doing legal

You only get one reaction so you can't deflect and redirect multiple attacks of opportunity, only one.

As AndredSkratch pointed out, once you take a reaction you can't take another until the start of your next turn, so you can only deflect one of those opportunity attacks. 

I don't think there are any specific rules for dealing damage by jumping on top of someone, so that part sounds like a homebrew thing your DM may have come up with? The Monk's Slow Fall feature should allow you to avoid taking any damage from it yourself at that level (since you would reduce fall damage by 25 and the maximum damage from a 30-foot fall is 18). One could reasonably argue that if you're falling slowly enough to avoid taking damage, you also wouldn't deal any damage to anything you landed on.

Also, Slow Fall doesn't stop you from being knocked Prone by the fall, though Tipsy Sway will let you stand up again more easily.

I am reading the rules, I am wrong about the deflecting stuff because I would have to use my reaction to do Slow Fall

This is from Tasha

• Falling onto a Creature
• If a creature falls into the space of a second creature and neither of them is Tiny, the second creature must succeed on a DC 15 Dexterity saving throw or be impacted by the falling creature, and any damage resulting from the fall is divided evenly between them. The impacted creature is also knocked prone, unless it is two or more sizes larger than the falling creature.

and this is from Xanathar

• Falling
• Falling from a great height is a significant risk for adventurers and their foes. The rule given in the Player’s Handbook is simple: at the end of a fall, you take 1d6 bludgeoning damage for every 10 feet you fell, to a maximum of 20d6. You also land prone, unless you somehow avoid taking damage from the fall. Here are two optional rules that expand on that simple rule.

and from the PHB

• Level 4: Slow Fall
• You can take a Reaction when you fall to reduce any damage you take from the fall by an amount equal to five times your Monk level.

So the monk would jump 30 ft up and then fall onto the enemy.

The enemy rolls a DC 15 Dex save

If the save succeeds (enemy gets out of the way), the monk take up to 18 damage, which gets reduced to nothing. According to xanathar you don't land prone if you take no damage.

If the save fails (enemy gets stepped on), the enemy takes half of the 3d6 and is knocked prone. The monk takes the other half but again, gets reduced to nothing.

Huh. Never seen that thing from Tasha’s before! Learn something new every day I guess. Sounds like you’ve understood it correctly.

Quote from babybabystewie >>

• If a creature falls into the space of a second creature and neither of them is Tiny, the second creature must succeed on a DC 15 Dexterity saving throw or be impacted by the falling creature, and any damage resulting from the fall is divided evenly between them. The impacted creature is also knocked prone, unless it is two or more sizes larger than the falling creature.

[...]

and from the PHB

• Level 4: Slow Fall
• You can take a Reaction when you fall to reduce any damage you take from the fall by an amount equal to five times your Monk level.

So the monk would jump 30 ft up and then fall onto the enemy.

The enemy rolls a DC 15 Dex save

If the save succeeds (enemy gets out of the way), the monk take up to 18 damage, which gets reduced to nothing. According to xanathar you don't land prone if you take no damage.

If the save fails (enemy gets stepped on), the enemy takes half of the 3d6 and is knocked prone. The monk takes the other half but again, gets reduced to nothing. 

There's an argument to be made that slow fall reduces all the damage, so there's nothing to split.

There's an argument to be made that you'd split first.

This is, ultimately, your DM's call.

The language of skill says "any damage you take from the fall", from which I read that damage is calculated, and then that damage gets reduced by the amount.

I think the name of the skill being "Slow Fall" makes people ignore the mechanics of the skill itself. The PHB simply talks about damage reduction. It doesn't say anything about reducing falling speed. Comparing that to the spell Feather Fall we can see that Slow Fall is a distinct skill.

Feather Fall : 

Choose up to five falling creatures within range. A falling creature's rate of descent slows to 60 feet per round until the spell ends. If a creature lands before the spell ends, the creature takes no damage from the fall, and the spell ends for that creature.

I think looking at the flavor of the Monk class, it makes sense

 A monk is a hand-to-hand combat master, and is someone that could jump up and stomp on someone. S/he isn't necessarily someone who can float slowly down to the ground, simply someone who is trained to land correctly.