You're almost right about the best possible CPUs, Apple could have released low end (2 core/4 thread) MacBook Pros with Kaby Lake CPUs, but I don't think marketing would allow it.
Apple only really cares about the immensely profitable iOS devices. The iPad Pro is too close to the price of the abandoned MacBook Air and the forgotten MacBook, and explains why there's no touchscreen and Apple Pen on new Macs.
About release dates, according to ark.intel.com all these CPUs were released in Q3 2015, except the i7-6567U which is still announced for Q3 2015, and the "i7-6660G" doesn't exist, maybe it's the i7-6660U, released in Q1 2016. Kaby Lake CPUs were released in Q3 2016, and you're right, they're the typical 2 core/4 thread i5 and i7's.
I agree that i3/i5/i7 are marketing names, checking CPU specs in Wikipedia or ark.Intel.com is necessary. Most mobile i5/i7 are similar to desktop i3, and in desktops/workstations quickly gets confusing comparing i3/i5/i7 with equivalent Xeon E3, and i7 Extreme with Xeon E5/E7.