Get(Platform) gives indication of the physical device type. Use the Get(SystemPlatform) result a a selector for WD layouts.
Get(Platform) does (to my knowledge) not distinguish between iPad and iPad Pro. Use Get(WindowDesktopHeight) to distinguish between the two.
Well, since we can't see the calculation for the $$PLATFORM variable, it's hard to say for certain. However, to your first issue, I generally avoid exact numbers for screen width or height. Despite the published statistics, I've run into issues with devices not matching. Therefore, I usually use something like Get ( ScreenWidth ) > 2000.
To your second question, you haven't told the system that there's any difference between desktop and web. Line 18 just says "Desktop/Web" and goes to a single layout. So naturally, it's not going to do anything different.
Also, it looks like the first selection in your Choose statement points it to Desktop/Web. Choose is a zero-based function, so if Get ( Device ) returns 0, it means "Unknown" - but you tell it to point it to Desktop/Web. You might also have "Desktop/Web" in the additional results of 1 and 2, so it might not matter - maybe you intend that layout to be the default? But if not, then you might have an issue.