1) The (~~に)たる is [足]{た}る/[足]{た}りる, meaning [十分]{じゅうぶん}な, "to be enough". → 足りる in デジタル大辞泉 #3
「(subject = あの方が予言した大魔闘演武の結果は、)それ(= 国の未来を動かす/あの方を信じる)に足る(= のに十分な)結果(= あり得ない、特殊な結果)なのです。」
(足る sounds more literary than 足りる.)
2) (~~が)ごとく(=如く) means (~~の)ように, "(just) like~~" → 如く in デジタル大辞泉
[螺旋]{らせん}が[如]{ごと}く can be rephrased as [螺旋]{らせん}のように.
(如く sounds literary/archaic.)
・・・ここでつまずいていらっしゃるんじゃないかな~と勝手に思って書きましたけど、的外れだったらすみません。
EDIT:
1)
それに[足]{た}る結果なのです。-- It is that special.
I think the translation is alright; it's saying "It's special enough for that -- It's special enough to make me trust *that person*, special enough to affect the fate of this country".
● In this sentence, the subject is left out; "an unbelievable outcome" i.e. "the results of Daimatou-Enbu which *that person* has predicted".
● それ(≠It) refers to あの[方]{かた}を[信]{しん}じる = for me to trust *that person*, and [国]{くに}の[未来]{みらい}を[動]{うご}かす = decide/affect the fate of this country.
● [足]{た}る means "to be enough", so it's saying "The outcome / The results of Daimatou-Enbu that *that person* has predicted would be special enough to make me believe in him and entrust him with the fate of this country", i.e. "The outcome is so unlikely to happen that I would have to trust *that person* and entrust him with the fate of this country if his prediction should come true."
2)
[様々]{さまざま}な[想]{おも}いと[思惑]{おもわく}が、[螺旋]{らせん}が[如]{ごと}く[入]{い}り[乱]{みだ}れる。
Various thoughts and expectations are jumbled together like a spiral
I think it's alright, I'm not sure it's the best translation though... because I don't really understand the nuance of "jumbled together" (Sorry T_T). I think the 入り乱れる here is like [絡]{から}み[合]{あ}う, "to get involved / intertwined" →絡み合う on 研究社新和英中辞典
● I think the word [思惑]{おもわく} is used negatively, like... "ulterior motive" in Weblio英和対訳辞書