few questons

wow thx

1. Game Modes.

I would use multiple layouts and event sheets. You can make a base event sheet, and then additional ones for each game mode. Using different layouts will keep from getting cluttered and confusing yourself as well.

2. Dynamic Environments.

Well, for climbing on a wall, I would make the walls you want climbable sprites, and then have events to detect if the player is over these, then activating a group of events which simulate the climbing. Deactivate the group when the player leaves the sprite or dies, etc.

3. Sprites vs TileBG/Box/3Dbox/etc.

Sprites are the main interactive object, yes. But you can still apply movements and all that to Box, etc. TileBG is a cosmetic object mostly.

4. Can someone explain what's going on in this cab?

Will look at it. Someone may beat me to it though.

For the game modes question, it depends on if your objects behave the same and if you want more than one map. For example, if you have a top down shooter where the players/objects/scenery all behave the same in each game mode and the only thing that changes is the objective or rules, I would use different groups for each game mode in a single event sheet and deactivate/activate as nessecary, this would allow the same code to be used on each map/game type you make (just use the same event sheet). Maybe use a global variable to store which game type so you dont need to edit the event sheet for each map?