There is a bug with the l4d2 game. The datatype type used for cvars (such as sv_steamgroup) within the l4d2 game engine, does not support values higher than 16,777,216 by default.

When you have a steam group id that goes over that number, it most likely loops back to a lower number to prevent a stack overflow error. That would explain the incorrect steam group.

The game itself only supports 1 steam group. It's not physically possible to have more than 1 steam group added to a server in this specific way. While it's only possible to display 1 steam group join at a time, you could display a different one every time the map changes for instance.

No, this plugin just fixes the value of "sv_steamgroup", which only supports 1 steam group.

Did you retrieve the steam group id from "edit group profile"?

If this does not resolve your problem, I would recommend using the debug variable to print a message to chat when Event_TankKilled is fired. This will help debug your plugin.
And etc. for all the if statements. Then find where the plugin is failing.

If the event is never fired, it may be a faulty game event. There is always a few of these on sourcemod. Anther way would be to hook when a player dies. Then find a way to see if they were a tank or not. There is a victimname string value which might help. Otherwise, you'll have to read a prop entity to see if the zombie was a tank, which is a little bit more complicated. https://wiki.alliedmods.net/Left_4_Dead_2_Events#player_death

The default EventHookMode is EventHookMode_Post which doesn't support return Actions. You need to explicitly specify, EventHookMode_Pre when hooking the events.

You must install the latest version of the steamworks extension for sourcemod.
http://users.alliedmods.net/~kyles/builds/SteamWorks/

Then you can take a look at this plugin, to check if someone is in a steam group.
https://github.com/stickz/Redstone/blob/master/addons/sourcemod/scripting/nd_swgm.sp

Then you need to modify the plugin with the commands, to include and use the swgm library.
https://github.com/stickz/Redstone/blob/master/addons/sourcemod/scripting/include/nd_swgm.inc

Try disabling sourcemod. If the issue persists, it's likely a VPS issue. I would not recommend hosting a game server on 1 vCPU. It's always best to keep your CPU usage bellow 50% on VPS.

Other users on the VPS machine can effect your performance. Having 2 vCPUs helps a lot. There are also multi-threaded tasks like database querying on sourcemod, which benefit from 2 vCPUs.

Your lag is most likely shared VPS resources.Try upgrading to 2 vCPUs and see if that helps.

I would recommend a t3.micro or a t3.small (if you need more ram/cpu). https://aws.amazon.com/ec2/instance-types/t3/

Is your server hosted on a VPS or a dedicated machine? How many players are you hosting?

It could be a sourcemod plugin causing server lag. Try disabling sourcemod see if that helps. Then add plugins back - one by one, if you're unable to point the cause.