Minecraft Java Edition – Turning a Shulker into a Vehicle by Teleporting It

Here is a method I came up with while pondering your question, be warned though, I haven't had a chance to test it out, but I think it might work. Here are the commands and command block setup:

1: /testfor @e[type=snowball]
2: /execute @e[type=snowball] /tp @p <x> <y> <z>
3: /setblock ~ ~ ~ redstone_block 0 destroy

Remember to put numbers next to the ~ symbols to tell the command where to place the redstone block relative to the command block so it's constantly running the command in the next command block. Now here's the setup, please note that being able to to what I am telling you to do does require basic command and redstone knowledge:

1 = command block #1
2 = command block #2
3 = command block #3
[] = redstone comparator
{} = redstone repeater with a 2 tick delay
# = redstone block

3#1[]{}2

Also, you might want to run this command as to not lag your server:

/gamerule doTileDrops false

I also forgot to mention, you must be playing in snapshot 14w21a or later.

You don't have to use a selector for a playerscore. You can use any name you want, even if it's not a valid player's name. For instance, the following sets the score of a fake player named "pool":

/scoreboard players add pool money 10

Do keep in mind that if a player named "pool" joins, they will essentially "own" that score. You'd want to be careful when manipulating the scores in case their existence causes a conflict (such as running /scoreboard players add @a money 10). If you prepend the name with "#" (such as "#pool"), the name will not appear on the sidebar and also prevents naming conflicts because real players cannot have the "#" character in their name.

As for your mechanism, you'll want to use Chain command blocks, which directly specifies activation order, instead of comparators, which does not have a guaranteed activation order and causes delay. The delay can cause the mechanism to lock up if a player kills a zombie before the comparator turns off.

/testfor is not particularly useful, especially in multiplayer situations. You can replace it with a scoreboard operation that would account for multiple kills in the same tick, rather than assuming there is only 1 kill amongst all players.

Image example of the mechanism:

Commands:

1. Increment the fake "#pool" player's score equal to all playerscores for the "killedBasic" objective (or change "#pool" to a name you would prefer, removing "#" so that it appears on the sidebar).

   /scoreboard players operation #pool money += @a[score_killedBasic_min=1] killedBasic
   

2. Set playerscores for "killedBasic" to 0 like usual.

   /scoreboard players set @a[score_killedBasic_min=1] killedBasic 0
   

This is a multiplayer-friendly and multi-kill friendly solution that will not lock up if multiple kills are made in subsequent ticks.