If I compile the game to web, can this run on mobile phones? What other platforms can it run on? Could I also download the page and play the HTML game while offline?
All modern desktop browser should be able to run the games. ( And the games can be integrated into various platforms supporting HTML5 games such as Facebook, Kongregate… )
Modern handheld devices ( iPhone, iPad, Android tablets and phones ) should also be able to run the games: The touchscreen is supported by the games. ( Just use the mouse related conditions: When the user will touch the screen, it will be considered as a mouse click ). I’ve managed to play to Angry Peas ( compilgames.net/games/AngryPeasExample ) using Chrome on my mobile phone.
It should work on some browsers ( Firefox ) but I know that some others are restricting the use of local data so it is possible that the game won’t load. But maybe some tweaking will allow to use the game offline on any browser.
I wonder if its possible to create a stand alone game on android and ios with Game Develop soon?
You can try to use CocoonJS ( ludei.com/cocoonjs/ ) to package your game as a native game, but for now there must some incompatibilities. It should work smoothly in the future. 
What about multiplatform multiplayer? Can one play a level on android with other people using a iphone?
No, multiplayer is something that is hard to done right because it depends a lot of the type of game you’re creating, and you need strong knowledge before being able to do something properly. And moreover, for now the technologies and tools to do real time mutliplayer game with HTML5 are quite young and not well supported.