You can put any value you want in the export panel if you think it is not there. For eg. like you mentioned you are seeing just 2x, 0.5x etc. If you type in 10x and export, it will actually work.
Coming to your other point, it is bit difficult when you design in higher resolution and want to export it for lower ones. For eg. you have an icon on iPhone 6 plus (@3x) of size 150px. You already know that it should be of 100px for @2x and 50px for @1x. But to achieve that you will have to do something like following image which is less ideal.
But if you have it designed at @1x and your icon is of 50px, it will be simple to scale it up: