First your random problem:
Else is a little bit tricky. The event sheet is read and each event executed in order from top to bottom, as long as the conditions match. Else means only run this if the event previously did NOT run (unless you have a series of elses, but that works as an extended if-else chain). So the way it is now, lets say your random picked 3, which would be dirt. The dirt event runs. The grass event does not run. The tree event DOES run because the grass event didn't, so it immediately covers your dirt with a tree.
To solve -
Explicitly define 4<=rng10<=6 in event 7
Or Add Else to event 6 as well, to make it a proper if-else chain.
Your terrain manipulation problem is similar. When you touch grass, event 12 runs and creates a tree. Then event 13 runs because a tree is now there where you touched, and then turns the tree to dirt. This is another bit of learning curve for the event sheet system because Triggers (the green arrow) don't follow the normal flow of top to bottom, and will run whenever the trigger happens. Thus you can't use Else with a trigger.
This is a little troubling because when you turn one object into another, triggers will run on the new object and I can't think of a solution off the top of my head. The solution would be to use another system/approach, which I hate to bring up because you would have to mostly start over, but here goes. This is how I would normally approach a project of this type. Instead of using a unique object per tile type...
1. (Better) Put all your tiles into one object called "Tile", and control look and properties by animation frame and instance variables. For example On Clicked Tile, with subevents If animation frame=1, do something, else if animation frame=2, do something ect. On generation, usually you want to save the tile x/y position as instance variables, and any other properties/identification you like. The type of tile is determined by the animation frame number.
2. (Best) Learn/use the tilemap object and build a tilemap system. They are made for tile based games.