Honestly, it's possible, but I've never seen the show, and I don't know if they break down the corporate structure.
Owning a majority of the stocks in a corporation, doesn't automatically mean you have any real power.
Having the majority of shares, even if they are voting shares, doesn't mean you are part of the board of directors. And if the shares that MC held were retractable and redeemable, even if they are voting shares, then the board can force the shareholder to redeem (aka sell) them back to the corporation, often at a pre-set price.
"But, the voting shareholders decide who's on the board, and he had the majority of the shares!" I heard you say.
Again, does he actually have voting shares, or just the majority of the shares issued?
Do we know what constitutes a quorum of the shareholders according to the bylaws of the corporation? AKA if he has 51% of the shares, but they need persons holding at least 60% of the voting shares in order to hold a vote, can he get enough shareholders together to hold a vote to change the directors before the directors vote that need needs to redeem his shares?
Was he completely forced to sell/redeem his shares, or was his officer role just removed? (aka, he's no longer the President, the Secretary-Treasurer, or whatever). Shareholders are a step removed from choosing who the Officers are. Generally, Shareholders choose the Directors, the Directors choose the officers, and the Officers are the ones who issue the shares (even though they are often compelled to sign the share certificates by the Directors, the bylaws, or other resolutions).
So if he owns the majority of the shares, and was an officer, then he could very easily be removed as an officer, assuming that there wasn't a stipulation in his original appointment. Again, I would need to see the articles, the bylaws, any Unanimous Shareholder Agreements, resolutions, and other agreements.
Also, everything I wrote might not be valid in your jurisdiction. Don't take the late-night ramblings of anyone online as legal advice.
He was the Chair, the board "voted" him and 2 others out (somehow). The show states he got the controlling majority of voting shares from his father (also makes little sense since he was assumed dead but that is another gripe). He did not sell any shares as he is later returned to his position, this was just bad writing so they could have him be "poor" for a bit. This was a plot point about how it was his company but then written so poorly they seemed to forget how they set up the fictional company (ie. a private company where the majority of class A shares is held by one person). This would be like if elon musk (Kirkland brand Dr. evil) was booted from space X, everyone would be super confused as to how.
So yeah, this was the end of comic book movies for me, it killed it. They wrote something so bad that the only thing that sticks with me to this day is the hot mess of corporate structure.
It probably was just bad writing for the sake of a forced story line, but the board can (generally) vote out the chair, and the shares going from a deceased person to another entity is called transmission. Transmissions do generally require the directors to sign a resolution or two though, and if they voted him out as the chair, then they likely would do what they could to prevent the transfer of the shares.
And even if they booted him as chair of the board, it likely wouldn't have much of a financial effect.
That is great but you can't move the shares to another dead person (in the show both he and his parents are assumed dead for years) so we are meant to think that they where held in trust, for who? Also while yes the board can force out a chair, but in the case of owning a controlling majority of the business (as stated) this would be pointless and silly, like in my Musk example the entire board would be sent packing. The writing was so poor that it strained my confidence in my own knowledge of basic corporate structure.
Sorry, I thought the person who was assumed dead was the father, not the character receiving the shares. If the person receiving the shares was the one assumed dead, it makes way more sense as to why it was less believable. It's still possible but, unless things were set-up in a way that already assumed that everyone would be dead, except for that one guy, who is still totally alive but is going to be missing for a while, then it's highly unlikely.
Then again, this is a universe where magic exists, so a wizard did it.
Depending on how long MC was presumed dead, and if there were any issues with either his estate, or of his father, the shares could be held by the personal representative of either estate for quite some time. If there was any less than 7 years between when his father died, and when he came back, then it's completely reasonable that he would be entitled to those shares. The longer it is after that seven years, the less likely it is that he would have the shares. The shares could've also been held by another entity, like a family trust or holding company. And as long as a trustee, or a controlling interest in the holding company, was still alive and willing, they could give him back the shares.
But:
He wouldn't have an officer or director position in the main company if he was presumed dead. Directors need to be re-appointed every year, and depending on the set-up of the corporation, you sometimes need to be a director in order to be an officer, or the officers need to be reappointed every year. A position like "chairman" was likely appointed to someone else rather quickly after the presumed death. Then again, it might also have the wildest bylaws ever written, likely by a wizard.