This mod conflicts with the L4D2 swep pack (One from silly)

I will look into it.

What happens exactly? I can't find a clue.

The game crashes when i had Sillys SWEP pack but if either the L4D sweps or Sillys Weapons are disabled, the game works fine.

Then you have reached lua limit.
Desuscribe from some addons and install them again, and tell me if it crashes.

Okay, here is a bug I found that really annoys me. I attempted to use this Swep pack in conjunction with the "Drop Weapon" addon, and it glitched the normal HL2 guns out of seriousness. Things like parts missing, retarded ways of holding the gun, and the SMG shrunk itself into a speck of dust that now floats above my character's thumb. If you can, fix it.

I use both, I could never recreate the bug. No idea what you're talking about.
Ask the drop weapon developer.

Which Drop Weapon Command aree you using? I am using the one by Woi and akakakakakakak.

when in a multiplayer server, the weapon dissappears

Post a lua error.

No idea what your problem might be, check if other addons are conflicting.

The Light Machine Gun reloads in a silly way, the box doesn't come out and the animation looks like the one from the HL2 SMG.

Silly lost the old files for the addon. That happens because the box isn't connected with the magazine bones of the weapon and it looks weird. Blame Valve.

You can reload the weapons while looking down the sights, it looks really buggy, can this be fixed?

Eh I suppose.