"Normaliza" the score

Welcome on board.
Have you tried using the plugin:

It's not what you ask but it could help.

Hi,

You want one of two things.

Either transpose the score to the key of C (no accidentals),
or simply delete the key signature entirely, forcing all accidentals to show up in the score.

I think this second option is what you want.

Perhaps https://musescore.org/en/handbook/accidental#respell-pitches? It sometimes does things you don't want, so check carefully afterwards in case you need to undo.

If you wish to eliminate the key signature and have the accidentals placed on each note, simply click on the key signature you wish to get rid of, then double click on the C major key signature in the Key Signatures palette. The notes on the staff will then display any necessary accidentals (which were formerly in the key signature).

Here's the result:

Attached is a score for you to try yourself.

Also, you wrote:
In ex. a Bb variation in the key means that in the score is written the note B but I've to play the note A#.

Technically, with a Bb signature the note 'B' is written, and a 'Bb' is implied - not an A# - so if you change to C major key signature, a Bb will be shown on the staff.
Of course, though, you can always change it to whatever you want...

Regards, and welcome aboard!