Minecraft – How to program a command block to teleport if there is exactly one player nearby

Any selector arguments you use in testfor can be used in tp. Like so:

1.7 to 1.12: /tp @a[score_HEALTH=1] X Y Z
1.13+:       /tp @a[scores={HEALTH=..1}] X Y Z

There are problems with using low health to check for death. Instead, I'd recommend adding a deaths scoreboard:

/scoreboard objectives add deaths deathCount

This score will get set to 1 when a player dies (automatic, because of the type of objective), your commands can teleport players with a deaths score of 1 and then set it back to 0 (so the player isn't repeatedly teleported).

The following commands are a full solution, including the team score incrementing, for 1.13+:

tp @a[scores={deaths=1..}] 73 10 31
execute as @a[scores={deaths=1..},team=red] run scoreboard players add blue points 1
execute as @a[scores={deaths=1..},team=blue] run scoreboard players add red points 1
scoreboard players set @a deaths 0

This is a good place to use scoreboard tags. This will allow you to tag any player that has diamond and use that tag to target them within the tp command.

Use 3 command blocks in a chain. The first command block removes the tag from all players. The second command block tags any players who have a diamond in their inventory. The third command block teleports any player within the boundaries of the target selector who do not have the tag.

These command blocks have to remain loaded. I recommend putting them in the spawn chunk. I also recommend stopping command block output to prevent chat spam for op players. Use command:

/gamerule commandBlockOutput false

The first command block is RepeatUnconditionalAlways Active with command:

scoreboard players tag @a remove Obtained

The second command block is ChainUnconditionalAlways Active with command:

scoreboard players tag @a add Obtained {Inventory:[{id:"minecraft:diamond"}]}

The Third command block is ChainUnconditionalAlways Active with command:

tp @a[x=X,y=Y,z=Z,r=R,tag=!Obtained] ~ ~ ~2

Replace X, Y, and Z with the coordinates of your gate area. Replace R with the desired radius for that area.