Learn English – “By now” vs “now”

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.

There's an interesting semantic implication in shifting the position of now.

Consider this:

This feature is now disabled.

One would infer that the feature was probably destined to be disabled and now it has been done.

This feature is disabled now.

One understands that the feature is currently disabled and may at any future time be enabled again.

The real reason is that in the first case, the subject or main reference is the status ('disabled'); in the latter, the context ('now'). Note that the above inferences are drawn based on convention and not grammatical rules.