I thought it was a good question. It would probably be better for me to ask more experienced people for this, though. I'm kinda doubting it since this is 1player, but you never know.
I've heard of a few collabs before. One way I can see it working could be with one person designing graphics, another music, and someone does the main programming. Then you could share the files for each element for the program with the person doing the programming so they can work off of that.
To share the files with the other collaboration members, you'd just have to publish the files and find some way to get the key to the intended recipient.
Pretty much what he said, Im actually making a horror game myself. Inspired from mad father, Ib and the witches house. I have Rpg Maker, Im just being lazy right now lol.
Sure! Its similar to ib with the different endings, good, bad or middle and it revolves around a limbo like setting with spirits. Im working on character sprites as we speak. Hopefully it becomes popular lol.
If you are making a game that will be free, there is a huge choice of game engines. You can even use some of the high end 3D game engines. If you plan to sell your game, some game engines are available with no risk contracts. You only need to pay when your game starts to sell.