Isn't this an excerpt of a dictionary definition, like what this blog cites?
In this case, it means:
what one thinks/feels about a certain matter (which has taken shape enough to be expressed/presented/published)
As you have correctly translated, 程度 means "(to) the extent/degree", or you could take Vできる程度 ...
The "、" clearly shows that the whole 「都合の悪いところは破っておいた日記と一緒に持ち帰った」 is a long relative clause that modifies 小型デストロイヤーの足.
"... the destroyer's foot, [which I brought back (from the quest) together with the journal...]"
Without the 「、」:
It can be ...
Simply, it is な used as part of a na-adjective. Without a comma, do the following sentences make sense to you?
あなたは寿司が嫌いなようだ。 You seem to hate sushi.
試験は簡単なようですね。 Looks like the exam is easy.
If yes, the な in question is exactly the same. Of course, normally, no comma is placed before な, but since this 嫌い is modified by a relatively long adverbial ...
However I initially understood it as: "Most of them are experts in using magic"
I don't think you are wrong. Actually you got the gist except the actual sentence technically doesn't say so. 大抵 is basically a quantifier today whose core meaning is "most times/cases (of)" instead of "most part" or "most people". But when you catch any random guy and ask them ...
The が here is kind of a softener/bridge/segue to the actual relevant clause which follows the first. Oftentimes this is conflated with が meaning 'but', however it is not always appropriate to translate it thus. You will see this use of が quite often in sentences such as すみませんが、トイレはありますか？ 'Excuse me, is there a restroom (here)?'. This site describes it as an '...
My understanding is that both parts of your sentence describe the もの mentioned at the very end of the sentence.
A thing that collects the things felt about a certain matter.
A thing that collects (something) to the degree of being able to present (it).
The simple answer: the に in 発表できる程度にまとまったもの allows the adverb of degree 程度(extent) to modify まとまった.
発表できる程度 translates to "the extent it can be presented".
まとまる in this context translates to "to come together; to cohere".
In order to say that ある事柄について感じた事 has まとまった to 発表できる程度, we use に.
becomes, in English:
This が is used to introduce a new thing into the discourse. It's one of the most basic functions of が, and I believe you have learned it before. It roughly corresponds to "a" as in "a good restaurant". See: What's the difference between wa (は) and ga (が)?
Would は be indicating a contrast?
The sentence in question has nothing to do with a contrast. It'...