With Swift, we lost (easy) access to the various path helper methods of NSString. Of course you can still get at them by casting to NSString but how about a more Swift friendly library for working with paths? FileKit by Cihat Gündüz is exactly that and adds even more functionality than the old NSString methods provided.
Can you include Swift code in your Android app? Yes, you can. The flowkey app is mainly hybrid, but some of the code needed to be written natively to keep performance up. This article gives a bit of background and is then followed by a couple of more technical articles on how they did it, and how to use the JNI with Swift.
How are you gating code for different OS versions? There have been many techniques for this over the years so it might be worth brushing up on them. Of course, if you're in Swift it's easy with #available but things aren't quite so clean in Objective-C.
Can you use your Core Data model in a Swift playground? Kinda. Andrew Bancroft has a walkthrough of how to get things up and running.