I do the light ballbearring trick into the mist. Then I poke the end of my crowbar into the slot.

Nothing happens. The ballbearing disappears behind the mist and the crowbar comes back intact 

 "Mmmmmmm..."

I scratch my head scales in thought.

I light the candles and wait for a result.

Then search the skeleton without touching it. 

12

You light the candles and the area fills with dim light coming from them. Then you inspect the skeleton but you don't notice anything out of the ordinary. 

"Perhaps the slot is some sort of key hole." Lumiere suggests. "I suppose we are meant to find something that would fit within that gap to unlock whatever lies beyond."

Here's an image of the chapel

I stick a gold piece in the slot.

Ubogri returns to the door between purple and brown and attempts to open the door, grumbling, "Stupids probably got themselves killed."

 "OI Ubogri!! Where did you be running off to?!"

Jerigg takes a gold piece and puts it into the slot. You all hear some machinery moving from behind the wall, then silence. Noting happens. 

At the same time Ubogri goes back to open one of the doors. He takes the handle and pulls it revealing a stone wall with a spear coming right at him at incredible speed. 

Attack Roll: 28

If it hits then it does 15 damage

Ubogri groans as the spear piecing his stomach. He curses in orcish and carefully wraps his hands around the spear, pulling it out. He then inspects the wall that was uncovered by opening the door.

Investigation: 24

The spear is stuck in the wall, connected to some mechanism. If you close the door the spear gets back to its place. You can roll me a strength check to try pulling it out. 

 Lumiere, seeing Jerigg place the coin into the slot has an idea. "If a coin would go in there, wouldn't a loop of magical metal also fit? Something like a magic ring." She then realises she doesn't own a magic ring. "Perhaps there is a ring available somewhere we can try in the slot."

"The riddle mentioned something about not having to sacrifice in the red sphere tunnel if you had a loop of magical metal . . ."

Strength: 17

Ubogri pulls and pulls on the spear, but it is beyond his capability to remove it from the wall. He walks over to the other door and, taking better precaution to avoid another form of retribution, opens the door from the side.

"We not getting nothing done, we can be thinking puzzle like solutions while resting yes?"

Before taking a long rest I will use Divine Sense to check the room for good or evil or undead or celestials or fiends or other stupid stuff. Might as well use it since long rest will get back my charges.

I am taking a long rest to recover spells. A long rest will also heal any damage we may have taken. 

"Maybe some somebody can use piton and hammer to poke hole in gold coin. Then is easy to enchant with light yes?"

Ubogri walk to the other door and tries to open it. Nothing different happens from the previous one: door opens and a spear comes out of the wall revealing nothing but a dead end. 

Jerigg, instead, casts a spell to reveal the nature of this place. What he finds out is that this place has a heavy aura of good all around it.

If you take long rests you need to tell me in which order you're gonna take guard turns. Other than that if no one disagrees with the idea, you're free to long rest.

Falmo only needs a short rest, so he would be happy with guarding for most of the time.