Rel does indeed continue to bring up the rear.  Occasionally stepping forward to tap a shoulder and provide Guidance (keep in mind that casting requires touch and she can only concentrate on it for up to a minute on one target only before needing to cast it again if unused.)

She primarily focuses on the walls and the path behind the party, in case someone or something is trailing them.

"I think we should proceed with stealth, particularly those in front.  If and when we arrive at running water, I have magic to Shape Water and briefly quiet it so if we wish, we can stand still for a moment and listen.  I can also make a Minor Illusion of additional water to mask our footsteps."

Rel Stealth if it matters (since she is in the back):  14

If others agree, she does cast Minor Illusion to subtly add to the sound of running water to obfuscate the sound of the party's advance.

Tex leads the group in the front, following Lana if she wants to scout ahead. He has either Lizzy or Zane on his side. Tex tries to move silently (Stealth: 14).

Zane will follow Tex and Lizzy.

Stealth check: 9

You all begin to move forward into the southern opening now, some quieter than others. Rel begins to create magical rushing water sounds from the rear, hoping it masks the sounds of the less graceful party members. 

The air is cool and damp as you walk through the more confined tunnel. Walking for about twenty minutes before the tunnel opens up again to a larger area. The sound of water does appear to be closer but still a ways off yet. A moment in between Rel's lulling river sounds, a clunk is heard as Bellamy's pack slips from his shoulder and hits the hard floor. A scattering of bats screech and disperse from the room ahead, though you cannot see which way they went as the darkness is still clouding you from seeing beyond your 60 foot range. Spending a moment to investigate this new open area in the caverns you walk around for a few moments and conclude that there are two paths ahead, one that leads to the west and one that continues south. 

Lana, with your passive perception, you can see that the western path is leading towards the sound of water. 

"My apologies as my pack slipped. At least those bats did not attack us" whispers Bellamy

Zane is trying really hard to map their location in his head and figure out if the sound of water might be the sea. 

He will ask Tex for the light once more and examines both paths for signs of movement. Perception check: 23.

"Lana, of course, shared her observations about the south tunnel. "If this is an underground river it could be used for transportation - in or out. I would check if it is."

She stepped into the tunnel looking for signs of anyone walking in there and followed the sound.

Rel taps Lana's shoulder once more, conferring divine guidance on the ranger as she scouts one of the tunnels.

"The bats may have just been bats.  Or there may have been scouts for the vampires among them.  Be ready."

Zane, you can tell that while both paths have been traveled, the one that Lana pointed to with the sound of water, is the more traveled path.

Lana, are you scouting ahead alone or are all of you going?

Lana scouts ahead of the group. Preferably not too far, but if they are not following, alone until she sees that running water.

Zane says, "I'll be at the back. We might have unexpected guests."

And he stays to see if others are following Lana or not, but in a way as to also keep an eye on the other tunnel.

Tex follows Lana in some distance, perhaps 30 feet, to makes sure he can assist her in case of need but hopefully not too close to give her away. 
(Stealth Tex: 18 and Lizzy: 16)

Rel also remains in the rear as she has been, though never more than 60' behind so she can still see Lana with her darkvision.

Watching the other tunnels as Zane is (until she has to move forward if Lana is leaving her 60' range).  Still concentrating on Guidance for Lana.

Bellamy follows Lana from a distance staying the same positions as before.

Lana begins scouting ahead with and the rest trailing behind, distancing themselves from each other as they go. You all walk down this tunnel for close to an hour before you can hear not only the sound of the much more prominent rushing water, but also the sounds of voices. Muffled and faint, mixed with the louder sounds of water but indeed voices.

Rel moves forward as soon as the voices are first detected and taps Lana on the shoulder, giving her guidance once more before falling back.

Wordlessly, she summons her Mage Hand to be ready, then begins casting Minor Illusion repeatedly to (subtly) increase the rushing water sound, 5%, then 10%, 15% so as to slightly mask the party's approach.

She whispers to the group:  "If it comes to a fight, we fight, ideally with surprise on our side.  But if it comes to parley, I suggest having Zane or myself speak for the party."  If it does come to that, she will switch her guidance to whichever of the two is doing the talking.

(Unfortunately, since the short rest, her Disguise Self has worn off, and Rel looks like Rel again, not the brown-haired woman in violet and gold.  Though she is open to suggestions depending on who the source of the voices turns out to be.)

Zane whispers, "Hold on, Lana. I can make you invisible for ten minutes so that you can figure out what we're facing."

Once the rest of the preparations are done, Zane will cast invisibility on Lana.

"Great idea, Zane." Tex takes position back in the tunnel that leads to the supposed cavern with water and people. He will remain out of sight and out of earshot until Lana reports back.

Lana waited for all helping magic to sink in, smiled when her body disappeared - it was a strange feeling - and moved forward towards the voices. She only knew (better say, only saw) that woman, so, right now her main goal was to remember the appearance of all speakers to describe to the group.

(Stealth: 12, though with all the help - invisibility and masking noise - can it be counted as "help" for advantage and additional roll?)

Quote from BlueBayou >>

(Stealth: 12, though with all the help - invisibility and masking noise - can it be counted as "help" for advantage and additional roll?)

 
(I know that RAW it doesn't say you get advantage on stealth while invisible but I always allow it. so go ahead and roll again)