I tried out your multiline expression in an Xcode playground (Swift 5.1.2). You separate at addition (probably also subtraction), but not at multiplication (probably also for division). Didn't try the comments, but, like C/C++, inline comments should be treated like whitespace during the lexical analysis, so, it should work.
Has to do with the associativity attributes of the operators.
Ths does not work as written (XCode 11.2 beta in playground)
let a = (b + (c+d)*e +f*g)
Get error: Expected ',' separator (last line)
This requires a space after + on last line, and then works.
let a = (b + (c+d)*e + f*g)
Even comments are accepted. Tested the following:
let a = (b // b is 18.5 + (c+d)*e // c, d, e are 1.7 35.3 310.8 + f*g) // f, g are 1.7 18.5
Gives a: 11549.55
So, what's the problem ?