This is a very natural question.
Vicino can behave a bit like a fixed expression in location phrases, especially in everyday use, so you may see it left as vicino.
However, you will also very commonly see agreement:
That version is also very natural, and many learners are first taught it that way because vicina clearly agrees with la boa.
So the short answer is:
Because of its role in the sentence.
Here, ancora is preceded by questa, so it is clearly a noun:
The word ancora meaning again/still is an adverb, so it would be used differently:
In speech, the stress also helps:
In normal writing, that stress is usually not marked, so context does the work.
Yes, it is understandable and basically natural.
That said, many native speakers would be a little more likely to say:
The two changes you may hear most often are:
So the given sentence works, but it is not the only natural way to say it.
Yes. The given order is straightforward and neutral:
But Italian can change word order for emphasis or style. For example:
That version shifts the focus toward the location first.
So the original sentence is the most direct structure, but not the only possible one.