(If the DM allows, yes.)

Yes. It is generally assumed the PC that spots where a hidden/invisible creature is also tells the others that didn't spot it its location unless the player controlling the PC says otherwise.

(Okay, and I forgot to ask this too as it may affect Devaru's action, but after getting inside, how high up roughly does the creature appear to be? And how close would climbing the book case might get him to it?)

Climbing the bookcase would get you into melee range. The creature is on the ceiling so about 10 ft high.

Against Devarus better judgement, Gendan's call spurs him to dangerous action. That being the rather risky play of climbing a nearby bookshelf until he stood on level with the creature before leveling Scattershot. "It's time you return to the earth!" He declared as he fired off one shot after the next. Every shot carrying a seed of his spirit that would gnaw at the creature's attention until Devarus became the focus.

Movement: Further n and up a bookcase.

Action: Attack w/ Scattershot(melee)!:
#1 To Hit: 7 For: 17 non-magical piercing damage.
#2 To Hit: 15 For: 14 non-magical piercing damage.

If #1 hits, using Distracting Maneuver, inflicting 3 extra damage. Next attack by ally has advantage.
If #2 hits, using Goading Maneuver, inflicting 7 extra damage. Wisdom Save[DC 15] or it has disadvantage on all attacks not on Devarus.

(If it looks like scattershot did little to no damage, using free action to put it away for the turn.)

(Ammo deducted)

-

As Deva fires a volley at the foe (still invisible I assume) Runt aims the haft of her axe at where the creater should be, she takes a step back and readies her axe to swing. The tattoo on her shoulder begins to glow a bright blue white as sparks begin to flutter from her axe. With all her raw power, she swings her axe as a boot of lightning strikes forth at the creater.

Action: us all three of her charges to cast lightning bolt at the creature, avoiding allies as best she can.

DC 14.   21 damage. Half on save.

Evasion: 26

CON: 14 (Rule check error: this should be WIS instead of CON save)

WIS: 8

Climbing the bookcases, Deva hefts his scattershot and fires off volleys. The first attack hits and with a deep, rumbly and inhumane howl, the creature is knocked out of its invisibility, letting Deva take better aim and pepper it with another volley of darts. It's body seems to contort in unnatural angles to avoid the brunt of the second peppering from Deva. (Reaction: Uncanny Dodge on 2nd attack - damage reduced to 9 from 19) Runt steps up and sends a bolt of lightning hurling towards the creature. To her dismay, it dodges the bolt completely with inhumane nimbleness. (Evasion: Half on fail, no damage on successful DEX saves)

Elsara's turn.

Elsara, spotting this nimble creature and the party having a hard time getting to them moves into the room. She draws her blade and enters her fighting stance, spinning the blade as she utters arcane phrases, a ring of energy crackles into existence. She hurls it around the beast and attempts to pull it to the floor.

Bonus Action: Bladesong
Move: Towards the middle of the room, close enough to catch the monster.
Free Action: Draw Riley's Spirit
Action: Cast Lightning Lure. DC 14 Strength Saving Throw. On a failure, the target takes 6 lightning damage and is pulled to adjacent to Elsara.

Other Note: all allies within 30' of Elsara currently gain a +2 bonus to saving throws.

STR save: 22

Legendary Action (End of Elsara's turn): Twin Scythe Vs Deva

Scythe1 (+Critical Boost + Bleeding Strike): 19 Dmg: 9. Deva needs to make a DC18 CON save or take 1d4 Necrotic damage at the start of each of his turn. DC10 Medicine check as an action to remove.

Scythe2: 17 Dmg: 12

Deva CON save: 8 (DC18)

Lair Action (Happening at the end of Runt's turn because of convenience): Plunge into Darkness - Room is enveloped in Magical Darkness.

Creature's turn:

Scythe Vs Deva: 14 (Adv: 15) Dmg: 8
Scythe Vs Deva: 12 (Adv: 25) Dmg: 12

Grim Gash Vs Elsara: 24 (Adv: 21) Dmg: 16 CHA save: 21 (DC15)
- On fail, unable to recover HP until cured by lesser restoration

The creature sees the ring of lightning and its scythe glowing dark red, it slices the lightning in half. It smiles with crooked, rotting teeth and says, "Nice try... Now me." It remains suspended to the ceiling while swinging with both scythes against Deva's chest. How it remained suspended to the ceiling while attacking with both "hands" is a mystery. Must be magic. The creature with its deep, raspy voice cackles and magical darkness descends into the room like a cloud of black ink. In the darkness, Deva manages to deflect one scythe attack only to have a second one slash across his face. Sounds of clacking can be heard as the creature goes on the move. Elsara feels the dull tip of the scythe hook on to her shoulder from the darkness and slashes open a deep wound going across her torso. The scythe is rusty and dull but it cuts through her like she was butter.

Party's turn.

Can Elsara Counterspell The darkness?

Also concentration check on keeping Detect Magic active: 27

Unfortunately not this time.

Whilst she cannot physically see the creature, Elsara notes her spell list is limited. However, she can still sense it. She holds onto her blade, sheathing it in arcane energy to enhance it as she reaches up and slashes at the creature, leaping gracefully as part of her dance.

Bonus Action: Coat the blade via Shadow Blade + bonus feat.
Movement: reach below the creature, the ceiling is only 10 foot so just stab upwards. It didn't take a hide action, so we know its general whereabouts.
Action: Attack. Gain disadvantage for not being able to see the creature, but blade property grants advantage to attacks in dim light/darkness.

Scimitar vs. Monster: 24 vs. AC. Hit: 9 magical slashing, plus 2 psychic damage.
Scimitar vs. Monster: 25 vs. AC. Hit: 10 magical slashing, plus 12 psychic damage.

((I did not mention properly that the creature is no longer on the ceiling. After darkness it dropped down, slashed Elsara and went in the general direction of the door out of the library.))

2nd Attack missed. Creature has Psychic damage immunity so only takes 10 damage.

Legendary Action: Twin Scythe Vs Elsara.

Scythe1 (Critical Boost - crits on nat18-19): 19 (Adv: 11) Dmg: 10
Scythe2 (Critical Boost - crits on nat18-19): 18 (Adv: 21) Dmg: 9

Elsara stabs the creature in the head with her scimitar and immediately feels two one of its scythes hooking into her stomach. The creature cackles again and then rips Elsara's belly horizontally open. She loses consciousness shortly after and falls to the floor.

[Elsara on DEATH SAVES]

Runt, Gendan, Deva and Val's turn.

The creature is close to Gendan and Val.

Rection to cast Shield to stop the second blow from taking her down.

Concentration save for Shadow Blade (DC 10): 13

(TBH if I knew it was attempting to escape the room, may've tried a booming blade to punish it from attempting to escape.)

Elsara manages to block the second blow and stay standing.Though she probably can't take much more of this.

Gendan regrets giving up her Devil's Sight last time her patron visited her. She can sense the monster nearby, but cannot see it. She swings her "Booming" pact sword:

Attack 17 Damage: 9 magical slashing, 4 thunder. Take Unable to parse dice roll. thunder if target willingly moves before my next turn.

On a hit, she'll add Eldritch Smite (3rd level) for Unable to parse dice roll. force damage and the target is knocked prone (no save).

Don't want to destroy that amazing disadvantage roll, so I'll fix the rest here:
Take 10 thunder if target willingly moves before my next turn.

On a hit, she'll add Eldritch Smite (3rd level) for 20 force damage and the target is knocked prone (no save).