Character Creation Rules

Wait was my HP 9? If that's the case then my new total should be 15.

Rolling for hp upon hitting a new level is not mandatory. You can take the average, but should you decide to roll instead, you must keep the result even if it's lower than the average.

I'm fine with you using stuff from Xanathar's, but keep in mind there's also lot of extra stuff for Planescape, such as powers unlocked by joining certain factions and rising in their ranks. Some is tweaks from older versions, some is home brew and all of it is secret until unlocked and available via your actions in-game