1/C2 will scale the project to the viewport resolution by default, to keep the assets quality lower, you will need to set the window size to an height of 360 AND change the project propriety "Fullscreen Scaling" to low quality
https://www.scirra.com/manual/66/projects
2/all the mobile phones out there have a different aspect ratio, that means that you canot have one size fits all without dynamically designing some parts (basically, placing some elements accordingly to the positions of the borders of the screen for exemple, with a scale inner or scale outer fullscreen), there are a good chunck of infos on the forums you can gather, even though I prefer 4:3 myself, 16:9 is probably easier to use as a base for a mobile game.