"-How can I write a script to allow stuff like coins and food to be picked up (and show on screen how many coins I have)" To show on screen your coins, would have to be done by editing the source files. You would also have to create models for the coins and food, and use them almost like ammo.
"-is their a way to write a script to end one map and automatically start in another one (for a single player game)" I think that you could use a trigger and a line in your map .cfg file for that. look in the docs ( http://sauerbraten.org/docs/editref.html )
"-is their a script that'll allow a trigger to change skyboxes (for the effect of night and day) also to trigger fog and rain?" You can also set up triggers (or timers with the sleep command) for the skyboxes. the fog wouldnt be a problem, you could just include it in the skybox changing code. the rain, however, would be more difficult as you can not turn particles on and off in-game in singleplayer mode. you could probably code something for that in the source files.
"-How can I make my maps Show up in the main map menu (I tried the tut but it just added an extra tab)?" I believe you could just add your maps to the variables? I havent tried that before so i dont really know.
"-How can I make missons like go kill this many enemies of this type, collect x amount of coins or food ETC?" You'd have to do that in c++
"-How to make textures (ones I made Show up in the editing menu with the other texture)?" You can make the textures, with some kind of software (i dont really know). To make them show up in game, put them in a folder inside your packages directory. you could name the folder "scharnhorst" without the quotes of course. then, make a file named package.cfg and type "texture 0 scharnhorst/TEXTURENAMEHERE.jpg" for every one of your textures. then go in-game, do /exec packages/scharnhorst/package.cfg and your textures will be available in the F2 texture menu.
"-How can I change the music (I also spent a few hours yesterday trying to make it work by following the tut but no luck)?" You can make a script for that too, i think. you can play music with the /music command. i dont really know how that works.
Hope that helps.