By moving 15 feet to your left, Anakis, you get a clear shot over Torrent's left shoulder at the rear flank of the toad, freezing it's muscles as it crouches to make another jump.

Seeing Tarek running towards it from its left, it veers to the right but can only cover 10 feet with its jump due to its icy joints. It slips on the ice as it lands and struggles to regain its feet. Then it makes another 5 foot hop out onto the frozen marsh.

The toad sniffing at Tarek's javelin decides the genasi running past it is more interesting and tries to snatch Torrent with its mouth. Bite: 9

Fluin's toad continues to flee from him and farther into the wood, where it is obscured from view. It too seems to have lost its footing in the roots and vines, making a loud thrashing sound, but then goes suddenly quiet.

Fluin, no longer able to detect his toad's location in the thick wood, hears Ulben being outnumbered nearby to his right. He rushes to his aid, dropping his rapier and drawing a second handaxe from his belt, which he attempts to bury in the toad's eye socket. Attack: 16 Damage: 6

Meanwhile, the other toad tries to stop Ulben from spinning by biting him. Bite: 17

The toad's mouth gets around his shield, but Torrent is too quick to get caught, sidestepping its bite.

Blood oozing from its ruptured eye, the toad is felled by Fluin's handaxe and collapses to the ground. Fluin steps forward onto his vanquished foe's head screaming, "Die!" and tries to bring the handaxe in his other hand down on the head of the toad that now has Ulben grasped in its mouth. Attack: 17 Damage: 3

The toad's jaws clasp tightly around Ulben, inflicting 3 piercing damage with its needle-like teeth and immersing him in a bath of venom which does another 5 poison damage. Ulben is grappled and restrained by the toad, and it begins dragging him away deeper into the woods to try to eat him away from its attackers, although its movement is slowed by its burden as it crawls back 5 feet from Fluin.

Torrent, what do you do?

Torrent, seeing that another of the toads has been felled, grins savagely.  "Yeh be next beastie" he says to the one which tried to gobble his shield.  "I'm tired'a ya'll."

Torrent will use Thorn Whip again against the one that just tried to bite him.  Attack: 11 Damage: 2

Torrent's thorny whip startles the toad as it hits.

Tarek, the fleeing toad is now about 20 feet from you as it heads across the marshy, frozen field, trying to escape.

Aluzira, you can hear the toad dragging Ulben away now 25 feet from you.

What do you both do?

"Gah! Ulben! Hang on!" Aluzira says, running up alongside the toad and taps Ulben to cast Cure Wounds for 10 Hit Points "Stop eating people!" she yells at the toad angrily.

Tarek, it's your move. Apart from the toad you're chasing, you can hear Torrent and the toad he's fighting unseen in the woods to your right about 25 feet from you.

Ulben, Aluzira's spell pulls you back from the verge of defeat, and you can feel the healing power of her touch surge through you. You still can't move away due to the monster's crushing bite, but you can use your action to try to escape by making a DC 13 Dexterity (Acrobatics) check.

Tarek calls out to the rest of the party, "For Pelor's sake, we can't let any of them escape!" He continues chasing after the toad, and, assuming he reaches the toad, he'll quickly unsheath his rapier and take a strike at the toad's heart (Attack: 22 Damage: 8), seething with vengeance after the incidents of the last battle.

The toad glances back at you, Tarek, and emits a gurgling shriek, its eyes wide with terror as it feels the sting of your blade and continues to flee in panic.

Ulben, do you try to escape from "the jaws of death"? If so, describe your attempt and make a Dexterity check. If you succeed, you will be free to move. If you fail, you will be one step closer to becoming a toad's dinner.

Anakis, you are in the wood 15 feet from Torrent and the toad he's fighting. Torrent is providing the fleeing and visible toad (40 feet away) with half-cover which you can get around by moving at least 10 feet to your left. The toad that bit Ulben is behind you and over your left shoulder, over 30 feet away. The last time you noticed the other fleeing toad, it was behind you and over your right shoulder, over 35 feet away. What do you do?

(So I can't see the furthest fleeing toad (35ft) at this point? Is there a position I can move to so that I can see it/make shot on it?)

For clarity, we're talking about the toad that fled from Fluin. (The other fleeing toad is actually a little farther.) You can't see it or any of the toads in the dense wood because it's a heavily obscured area which blocks vision entirely, so when trying to see something (e.g. a toad) in that area you effectively suffer from the blinded condition. I'm sorry if this hasn't been clear enough from my descriptions, but since it seems we keep having to go over this, maybe this more explicitly rules-based description might help us get on the same page.

The only toad you can see is the one Tarek just stabbed because it has leaped out of the wood into the open marshy field where it is visible.

Even though you can't see them, you know where two of the remaining three living toads in the wood are (the one fighting with Torrent and the one trying to eat Ulben) mostly because of the noises they are making and can target them directly.

The location of Fluin's toad, however, is unknown to you. As I related in post #701, it moved farther into the wood and became quiet, dropping from your awareness (as well as everyone else's). When this happened, it was at the location I described, 35 feet away from you. You can't be sure where it is now, but you can infer what you will from its last known position and target whatever location in which you think it is. 

So, to answer your second question, no, there is no position from which you can see it, but you are free to change your location to improve your chances of detecting it (by hearing), or you can guess its location and target that if you wish.

(Sorry, the only reason I ask is because some of them have gone out into the open and others haven't, so was just trying to clarify that part. I get the obstructed vision/blinded thing though, just hadn't linked the toad that disappeared somewhere as the one you were describing at the 35ft mark, I thought it was one that was recently engaged.)

Anakis moves to make the best shot she can on Tarek's toad, firing a ray of frost to slow it down for him (Attack: 21 Damage: 2)

No worries. Tarek's toad is the only one that has gone out into the open. You have advantage on your attack, so you can roll again (one die only) to see if you crit.

23 to check if critical. 
(Edit - so close but no cigar!)

Ulben flails around kicking a screaming swinging his stick trying to get free.

9 dex check

($0π of @ b!+©#)

"Hang on there, little buddy!" Fluin calls out to Ulben. "I'm going to get you out of there!" He steps forward across the remains of the vanquished toad beneath him, replacing one of his handaxes in his belt, and attempts to grapple the behemoth toad before him. Strength (Athletics): 23

The toad resists Fluin's grasp. Strength: 11

Meanwhile, Torrent's toad refocuses its senses on the genasi and tries once again to consume him. Bite: 6

Tarek's toad, which is beginning to look worn down after being hit with Anakis' ray, starts to leap away from him, giving Tarek the opportunity to make a melee attack.

With the toad firmly in his grasp, Fluin uses his powerful legs to move forward 5 feet, dragging the toad with him and pulling it off Ulben who is thus released from its mouth.

The grappled toad, robbed of its prey, turns on Fluin instead. Bite: 13

Held in the gargantuan toad's maw, Torrent takes 10 points of damage from the toad's piercing teeth and 5 points of damage from its dripping venom.

The fifth toad remains undetected.

(Good Lord, he hates me)

Torrent, struggling constantly against this toad, screams out in pain as the toad crunches down on him. The scream quickly dies as he falls unconscious. 

Hearing Torrent cry out in pain before dropping to the ground, Anakin yells out, "Torrent! No. I thought you had him," she cries, but her face quickly freezes over with determination as she squares up to take on Torrent's assailant. 

Fluin bears the toad's bite through sheer toughness and shrugs it off, holding the beast firmly in his grasp.

I'll await Tarek's attack before continuing.

Tarek, seeing the opportunity, will take another thrust at the toad before it tries to flee. "I won't let you eat me this time," he sneers. (Attack: 6 Damage: 4)