Hey Beerkan
I got part of it to work in multiplayer with me hosting a coop mission.
Now when my friend is ejected he gets a parachute on his back and it opens fine (this was the part not working before), he lands, doesn't get damaged, its just atm all the backpacks don't spawn back on the back but are replaced by the parachute backpack on the ground. What i'm sure can be fixed with some more tweaking.
So instead of
_x addBackPack "B_parachute";// Add parachute on exit of _vehicle. AI will immediately open it, players can open it at any height above _chuteheight
i replaced it with
[_x,"B_LIB_US_Parachute" ] remoteExec ["addbackpack"]; (im using IFA3 parachute, but it was tested with other types as well)
I read remoteExec is used for functions and stuff on multiplayer so i got it to work with that it seems. Now so i was going to go through and change parts to use remoteExec.
Also instead of if (!isServer) exitWith {}; at the start of the script i just blanked it out with //if (!isServer) exitWith {};