I'm that mage, I was close to win the semifinal to got at least 2nd place, but I made a mistake and lose haha.
I have some pieces of gear that I usually change depending on what I face on duels, and the trinkets, to have a good mix between resilience, crit, MP5 and sometimes haste.
That build is specificaly made for me to play alone (duels or not premade bg), thats why I have Arcane Shielding 2/2, when I need to use mana shield I dont wanna oom in 2 seconds, and I play around with that trinkets or double Solace and some pve pieces with crit + spirit.
The key on this spec, is to know when to switch between armours to reduce a cc, to slow meles/reduce damage taken or go burst mode. As I have high mp5, I can keep switching without problems.
If you take a look on my second talent spec and glyps, thats a build to go to arenas/bg with a healer (idoneal its Dpriest because of dispells), so I dont need evocation and can focus on more crit. There is a screenshot on my stats when I play with a good disc:
https://imgur.com/a/HnnfM9W
Feel free to ask me whatever, I'm not a pro, but have been playing fire for ages.
I like to just swap armorus when need it (to a death coil or repentance for example), and sometimes its important to just keep mage armor as fire (fighting dks or hunters for me is a must).
Burning determination for me is a waste, as I usualy fake silences well enought to not get catched and fire mages dont need to cast so much anyway. If you are not used to fake or are new at fire, sure, u can take it.
Anyway, it just depends on your playstile and prefferences, the important thing as fire is the fun of it!