It's possible that nil is a noun here, but it's more likely that it's the adverbial nil, which would translate to something like "not at all."
B. Adverb. 1. Not (as a strengthened non), in nothing, in no respect, not at all: “me nihil poenitet,” Plaut. Bacch. 5, 2, 63 (etc.)
So the difference between the two would be the strength of the negation. ...
To express the (apparent) quality of something, only adjectives can be coupled with videri. A few examples from Cicero, respectively De Officiis and Brutus:
Cum igitur id, quod utile videtur in amicitia, cum eo, quod honestum est, comparatur...
Thus, when what seems useful in friendship is compared with what is virtuous...
[...] qui eum sententiis, ...
Prīmum nōn nocēre means "first, to not cause harm". Nōn negates the verb.
Prīmum nīl nocēre means "first, to cause harm to nothing". Nīl is the object of the verb.
(Note that both are infinitives rather than commands in Latin; the original context is something like "the first law of medicine is to not cause harm".)
This is an indirect question and indirect questions always use the subjunctive (also known as conjunctive) mood.
If you want more examples and details, please the linked discussion in Allen and Greenough.
Although not needed here, I thought I should mention a relatively common use of this mood where the reason is semantic rather than syntactic.
From Allen ...
I agree that it makes little sense for adhuc to modify the whole sentence ("she still wanted"), as her desire to kill Hercules was not mentioned before, let alone anything that would lead us to think it had by now subsided. My interpretation is that the adverb adhuc modifies the adjective infans:
Herculem infantem necare voluit.
She wanted to kill ...
No, it means that an organ is still (including today) not used in the presence of the pope.
One would have to change the tense of utor from utitur (present indicative) to a perfect tense, which shows the action was completed some time in the past: usus est (perfect) or usus erat (pluperfect).
Perhaps a clearer English translation would be:
the Roman Church ...
Ben Kovitz listed some relevant words in his comment and Sebastian Koppehel added some more.
In the case you mention ("this option is obviously correct") I think the most natural option is to use and adverb derived from an adjective.
The others feel too weak for such an emphatic use, much like "indeed" is weaker (and otherwise different ...
From Lewis & Short's A Latin Dictionary:
q. The use of ab before adverbs is for the most part peculiar to later
Latinity: “a peregre,” Vitr. 5, 7 (6), 8: “a foris,” Plin. 17, 24, 37;
Vulg. Gen, 7, 16; ib. Matt. 23, 27: “ab intus,” ib. ib. 7, 15: “ab
invicem,” App. Herb. 112; Vulg. Matt. 25, 32; Cypr. Ep. 63, 9: Hier.
Ep. 18: “a longe,” Hyg. Fab. 257; ...
Don't think of adhuc as "still" but as "up to/until here," or "up to/until now".
The goal of Steadman's commentary is to keep things very simple (it glosses adhuc as "still" with no alternatives). The formation of adhuc is ad + huc - to, toward + to here (in place or time).
Or perhaps you're dilemma is the tenses. ...
The word ante is both a preposition and an adverb, whereas antequam is only an adverb — or more accurately antequam falls under the adverbial use types of ante and antequam is essentially a conjunction.
If you need a preposition, ante is your only option.
If you need an adverb, the difference between the two is small.
In L&S both words are given under ...
In addition to the adverbial forms given by Joonas, you could also use an impersonal construction like this:
Patet X rectum responsum esse.
Or even more forcefully:
X rectum responsum esse solis luce clarius est.
It is clearer than sunlight that X is the correct answer.