I would like to use the coop's spawn and disassembler instead of the elevator in my maps. I've tryed to do it but no trigger wants to trigger on the bluebot_dropper instance and the console shows me that the game wants to connect to a server but I don't want it to connect to a server because it's a singleplayer mod (hit me if I'm wrong).

How can I do that ?

Thanks !

(and how could I replace Chell's skin with the Atlas's one in the mod ?)

First of all you need to edit valve's instance so then the player looks like they are being assembled/disassembled. Make sure you use a point_viewcontrol instead of a point_viewcontrol_multiplayer.
For changing Chell into Atlas, that is relatively easy. All you need to do is place a dynamic model anywhere in the map as the atlas player model and put a console command in (using a point_clientcommand) "setmodel player/ballbot/ballbot" . You will need to change the player model every time you load a new map.

I haven't tested it but is your model change technique replacing sounds and things like sparkles when the player hit a wall to hard ?

It will automatically take the animations from the bot player model (and sounds). There has also been a previous thread explaining this:

mapping-help/robots-not-chell-t3709.html

Hate to say but please search before posting. It angers me to explain the same thing several times.