neutral-description when X(が) is variously modified

Question

The following is from this guide to が versus は:

Below are subjects that are within the universe of discourse by default: [...]

Subjects modified with additional information, such as この / その / あの, 今日の, etc (because this additional phrasing introduces it first). [...]

「あの牛は草を食べている。」 (「あの」 introduces 「牛」 into the universe of discourse. If が were used here, it would be exhaustive-listing [...], which would be a rare case.)

Based on the above, my understanding is that a sentence like「牛が草を食べている。」is valid as neutral description only because the noun 牛 isn't modified by anything. (See this for "neutral description"). However, when 牛 is modified, there are cases where neutral description isn't normally achieved by が.

This is also discussed here (answer by user4092) where zero-particle (φ) is presented as the natural choice for neutral description when the noun being described is modified by この / その / これ / それ.

I still have a few doubts regarding non-deictic modifiers and differences to do with politeness levels.

Which option is the best to express neutral description in the following sentences?

1. あの牛 (が? / は? /φ?) 草を食べている。(deictic - the speaker is pointing at the cow)

2. あそこにいる牛 (が? / は? /φ?) 草を食べている。(deictic in noun-modifying clause)

3. 白い牛 (が? / は? /φ?) 草を食べている。(non-deictic modifier).

-For those cases where the zero particle φ is the best choice in daily informal conversation, please also indicate whether φ would be kept in formal writing, or whether a different particle would be chosen.

-(optional) I'd also appreciate a comment on whether the number of cows in the field - one VS many - would affect the choice of the particle, while still aiming at neutral description (no focus, no contrast).

Answer 1

It's too difficult for me to say something generalized, so let me focus on what's been explicitly asked...

Which option is the best to express neutral description in the following sentences?

Based on my intuition as a native speaker:

1. あの牛_Φ、草を食べている。
   _{(This is okay only in informal speech; in formal writing, people simply prefer Sentence 2.)}
2. あそこにいる牛が草を食べている。
   _{(Φ is okay in informal speech)}
3. 白い牛が草を食べている。
   _{(Φ is unacceptable even in informal speech)}

I don't know why we can drop が in Sentence 2 but not in Sentence 3; maybe 白い does not have a function of introducing something in the discourse?

whether the number of cows in the field - one VS many - would affect the choice of the particle

No. In all the three cases above, there can be one cow or several cows.