Which part of my statement applies to case #2?
In case #2 you can spend a lot of time developing the product, but in the end it might fail, just because the dependency fails. What if Apple changes iPhone UI structure completely? - you'll have to spend extra time updating the software. What if iPhone leaves the marked? - your the application is deprecated.
The reason why Symbian port didn't make it. Everything changes, you can choose to stay compatible or leave. No problem. GTK1 was the first Lazarus interface, now it is deprecated in favor of GTK2. GTK3 is on its way.
Another thing: don't forget that even developing for Android with FPC either SDK/NDK will be used! Why not to write our own Android platform (Cheetahoid)? Sounds crazy? risky? stupid? interesting?
Wanna do it? Feel free. Note that in case of FPC through arm-android target only needs assembler, linker and platform libraries. That's the same as the desktop targets. The SDK is required only to build the APK and debug it.
So taking the same approach for other tools (interface builder) also make sense
Again, feel free to do it if you can, but you won't ever make it to the source code repository. There are times when we have to invent our own, and there are times when we have to integrate with existing one. FPC used to use GNU as/ld exclusively, but seeing its overhead the plan began to write internal assembler and linker, which now exists for win32/64, dos and bsd targets. If Lazarus decided to build its own GUI backend when it was first launched, I don't think it will ever be as big as now.