Respawn dynamics of command blocks in a Minecraft event

What I would do:

Don't set their spawn in the sky, instead make a deathCount objective, and at the beginning of the 'event', set it to 0 for all players. When a player dies, teleport them back to the beginning, then set their deathCount to 0.

If you are doing a single player map, you could, instead of using beds for spawns, have a button that summons an invisible invincible armor stand, and then use that for the 'resetting' part.

If you are doing this with multiple people on a server, you could also at the beginning of the event, kill all players, then summon an armor stand at each player with a custom name, then use those to set the spawns back.

How to use /scoreboard

First, make the objective:

/scoreboard objectives add abc deathCount  

Then, make a clock going to two command blocks.
In the first block, put

/tp @a[score_abc_min=1] <x> <y> <z> (coords of intended spawn)  

The second block should have

/scoreboard players set @a abc 0  

abc can be replaced with whatever you want

New answser:

Use a command block close to the middle of the room, (could be on roof or in floor) on a relatively quick timer. Use /tp @a[rm=?] <x> <y> <z> to teleport everyone at least ? blocks from the command block to x y z.