Greetings,
1. http://www.wowdb.com/spells/21562-power-word-fortitude and http://www.db.pandawow.me?spell=183582/dark-intent
2. Should not stack with each others and with all stamina buffs.
They do stack as visual without giving any effects, but the important thing is that when you Purge a player that have all these buffs at once,
you gonna be forced to purge useless buffs which takes more time to purge the desired buff and consume mana.
So, these buffs should not stack at all and instead be replaced with any other buff.
I did report it about a year ago and got fixed, but now it got bugged again. (old report: http://forum.pandawow.ru/showthread.php?t=224918)
3. Realm: x100/Fun/Cross (tested today 8/15/2018)
4. 5/10 (Since purging is essential)