Unlocking the bootloader and Wiping Data

Hi,

I've did a little research about unlocking bootloader on the moto X Play and I found out that this process wipes all your phones data.

Question is....

Does the External SD Card remain untouched. So if I complete a Helium Backup and store that said backup on the External SD Card I will then be able to restore all my data back onto the phone?

As you may imagine the main purpose of the Unlocking Bootloader is to be able to root the phone and install custom ROM on it. The main issue I have with stock ROM is that the External SD card runs independent of the Internal Storage and I'm finding that I run out of room really quickly.

Is there a custom rom that will enable me to use MicroSD card to EXTEND internal storage?

Thanks