Okay! I finally found what it was!
The mod causing is the "defaultworldgenerator-port" mod (https://www.curseforge.com/minecraft/mc-mods/default-world-generator-port).
To enable the choice of a world type when creating a new world, check the link. The mod creator explains how to change the configs.
i hope this will save some time to people wondering what may have been causing this. unless I'm the only dummy around.
Thank you for your time and patience, I think we can call this case closed.
Thus, "voidislandcontrol" has nothing to dowith my 2nd issue. ; )