Learn English – Do “in future” and “in the future” imply different meanings

Neither is more correct. They mean different things.

You first example can be interpreted like so:

Now that I am the main stakeholder ...

Which means you are about to make a pronouncement about how things will be with you are the main stakeholder.

E.g.

Now I am the main stake holder, all developers will have their wages halved.

As a statement by itself it can be used to mean I am now the main stakeholder, but you have to put emphasis on now and make it seem like you are marking the the moment. A similar idea would be to make a future announcement:

From 6pm today I will be the main stake holder.

Of course, if you put a comma just after now you get a different meaning:

Now, I am the main stakeholder.

This is using now as an interjection, it doesn't really mean anything with regard to the sentence. The rest of the sentence is just a statement explaining who you are.

Your other example

I am now the main stakeholder.

is a statement explaining that from this moment you are the main stakeholder. This structure can be used in a triumphant way, as an exclamation, for example:

Jensen Button is now the winner!

or in a dry factual way:

I am now the answerer.

In your example, which is not a relative pronoun, but a determiner, pre-modifying boy. As a determiner, it can be used with animate and inanimate nouns. The sentence is grammatical, but it’s hard to understand what it means out of context. It would be more likely to occur as ‘I don't know which boy you are meeting.’

Unlike the first sentence, the second one contains not one, but two, relative clauses, but it, too, is a most unusual sentence. Its most natural manifestation would be as ‘I don't know who the boy you are meeting is’.

The first relative clause is ‘who the boy. . . is’. The second is ‘you are meeting’. This second one omits the relative pronoun, but if you want to include it, you can. In that case it would be either that, who or whom. A relative clause can be introduced by that when, as here, it forms part of an integrated relative clause, also known as a defining relative clause or a restrictive relative clause. The choice between who and whom, in both relative clauses, depends on the formality of the context, with who being less formal, and probably more frequent, but both are found in Standard English. In relative clauses, which acts differently from when it is a determiner. In a relative clause, it can only refer to inanimate antecedents.