Hey Xiao Wan,
Really glad to hear your enjoying the toolkit! 
To your questions…
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You will require a mac to build the project for submission to the app store or complete build to your devices for testing (Unless this has changed recently?)
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To deploy to an IOS device for testing you are required to be an apple developer. You can test your game in the mobile preview, but I would suggest signing up to the program as its really worth it being able to see your game running on device.
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You should not have any troubles deploying to an IOS device. Personally I do not have a mac, so I have not been able to submit any projects based off the toolkit to the app store. Maybe another user has had success with this?
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Multi-platform is a beast on its own really. Unreal handles most of the muscle work here to make other platforms accessible and deployable for almost all devices from the one project. Design wise, you will need to take into account that some platforms may or may not have the muscle to run as many visuals, code, post processing effects, etc per frame and performance issues may arise on lower end machines (Anything from assets, to complex code, post processing limitations etc), the list can go on for days. You have the option to branch a build and setup/ optimise the project based on the platform, however you shouldn’t do this unless its necessary (Such as porting from a visually detailed PC/Console game to a mobile platform)
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This is definitely possible, as likely expected you will need to modify the toolkit to meet these requirements.
Hope this helps