This is how I changed the default app. Hope it works for you :
A) "ok Google, set alarm"
B) the following screen appeared
X) tap on the icon and change the app to your desired alarm app (I use any.do as it has a snooze function)
I ended up reading this question after I also tried basically all of the steps / websites mentioned in the question. But I eventually managed to install TWRP.
In order to make the SP_Flash_tool start flashing, after you plug in your phone that is turned off, press power button + volume up
If the flash tool errors, you might not have the right scatter file ...
If you can get it into fastboot mode, here's how you can bootup your device:
Get Google's platform-tools (aka adb), from here
Boot your phone into fastboot mode
Run this command with platform-tools: