When executing slash commands via server.executeCommand in a server-side script, it requires the server to be in "active cheats" mode. Therefore also allowing players to execute arbitrary commands and putting themselves into gamemode 1.

An adventure map where slash commands by scripts are needed can therefore not use them without also allowing cheats for the players. The only workaround is using command blocks with pressure plates on top of them and spawning entities on top of those. This does not require active cheats but the server script can still execute slash commands with higher privileges.