When that old man recognized you last year, didn't I give you the money to keep him quiet?". I don't understand.. Why should Jerome give money to Santini to keep some guy quiet? I would rather think that he gave money to the guy to keep his mouth shut. It makes no sense for me
He's gave Santini money, that Santini then gave to the blackmailer. The blackmailer probably didn't want anyone else to know; Blackmail is a crime too. So the money had to come directly from Santini.
Jerome was going to unmask Santini. And what would be his plans for later if Santini is not there anymore?
Jerome would find another magician to take Santini's place .
"when Santini makes his first performance, there is one thing which always bothers me. He takes a red scarf/bandana/napkin (I don't know the proper English word),
and suddenly some jar with flowers appears on the table. So for us as a viewers it is a nice tricks. But what about the people who sat
behind him and could clearly see how he makes his trick?"
If you watch that scene closely, the lady to the right of the picture is clearly holding the vase of flowers below the table. For a very brief moment you just catch a glimpse of it.
When Santini waves the cloth and holds it over the centre of the table you can clearly see the lady lift the vase up and place it behind the cloth. Watch her eyes and head move as she does so and see her arm change position.
You are right though, anyone looking from any angle other than ahead would easily spot how that trick was done either without the female assistant.