m3741:
Actually, if you look at the function pmc_switch_sclk_to_32kxtal() in hardware/arduino/sam/system/libsam/source/pmc.c it does the exact same thing as the code you listed above. Since your answer is duplicating work, I fail to see how it is simpler.
When I first tried your suggestion, I put the function in the point you suggested in your first post, and it didn't work. So, I read the datasheet and found the register & bit that enabled the external clock and used it. It was "simpler" for me instead of looking the whole core trying to find what that piece of code did 
You are very correct that I was not waiting for the chip to complete the change. It did work fine for me without waiting for that bit
I agree. It can work (it worked fine for me too) but the datasheet "suggests" to wait the change so I think it's better if we "follow" that suggestion 
Thanks!
You're welcome 