Minecraft Java Edition – How to Remove Excess Arrows Using Commands

If you are in 1.8+, you can also use vanilla commands:

/kill @e[type=Item,r=500]

r is the radius. @e is entity.

As for your "Redstone doesn't work when far away problem", this also applies to command blocks. What you want to do is put all of your contraption in the spawn chunks or move the spawn chunks to your contraption with this command:

/setworldspawn x y z

This is because Spawn chunks are always loaded and the setworldspawn command can move worldspawn and thus also spawnchunks

Now with your problem with the fireballs behaving weird around other players in multiplayer. Maybe try giving fireballs their own score or tag which you can then test for and then make it so that only fireballs which haven't been shot yet reset the score. This is what I kinda have in mind:

/execute @a[score_Fireball_min=20] ~ ~ ~ /testfor @e[type=Fireball,r=3,tag=!notStationary]
/execute @a[score_Fireball_min=20] ~ ~ ~ /entitydata @e[type=Fireball,r=3] {tag:notStationary}
/scoreboard players set @p Fireball 0

Hopefully this explains my thoughts somewhat good as I'm not really that great with testfor and execute, feel free to call me out if what I am saying is bs tho as I am currently unable to test this as I'm not able run mc atm.