The accent helps distinguish è (is) from e (and).
This is a very common spelling point in Italian:
So in this sentence, the accent is necessary.
Dentro means inside or inside of.
In this sentence:
It is close in meaning to in, but dentro emphasizes being inside something.
For example:
Both can work in many contexts, but dentro feels a bit more explicit.
Both dentro il corpo and dentro al corpo can be heard.
That is because dentro can be used:
In simple learner contexts, dentro + article + noun is often the easiest pattern to recognize.
This is the normal Italian word order:
subject + verb + place
So it works very much like English:
Italian word order can sometimes change for emphasis, but this sentence uses the most neutral and natural order.
Sometimes yes, but the meaning shifts slightly.
In many situations they are very similar. However, dentro stresses the idea of being inside more clearly.
So:
No, not here.
Italian often drops subject pronouns like io, tu, lui, because the verb ending already shows who the subject is. But il cervello is not a pronoun ending built into the verb—it is the actual noun subject.
So you need the noun here:
You would not normally omit il cervello unless the subject had already been made clear in the conversation.