Janet is not an implementation of common lisp, so you cannot run lisp code with it. ECL is an implementation of common lisp, so it can run common lisp code.
If you don't care about running common lisp code, you can get up and running in janet more quickly.
In terms of ease of embedding in C code, a comparison of different languages IME would be Lua/Fennel > Janet > Chibi Scheme > Guile Scheme > ECL.
yes, objective C is a superset of C, so you can include janet's amalgamation follow the regular embedding steps. i'm sure you can get it running from swift as well.
If you don't care about running common lisp code, you can get up and running in janet more quickly.
In terms of ease of embedding in C code, a comparison of different languages IME would be Lua/Fennel > Janet > Chibi Scheme > Guile Scheme > ECL.