They threaten the old guy to hand over the Casino in the beginning?

They could have but the Oceans team knew that to get back at Al Pacinos character they needed to hit him where it hurt and then some. Yeah they could have strong armed him to sign over his shares or money but to a guy like Pacinos character that's slim pickings since he has a lot of casinos. What mattered most to him other than money was his reputation as a respectable casino owner so that's why they went with not only breaking his bank on the soft opening and letting the guest walk out with all their winnings but stealing the diamond awards from his other openings which was basically his reputation as the best casinos in Vegas but also wrecking the one guy's stay and having everyone treat him like dirt when he's the one that will put him on the map as the best new hotel and casino and get him the diamonds award again.

See to him it's all about reputation and those diamond awards; the money was just to rub it in too. Bankrupt him, steal the diamonds and ruin him.

It's easier to get someone to sign a contract in an isolated place of your choosing (a construction site) than to threaten him to give up his PIN and card in public, and even then most accounts have daily withdrawal limits.

In the first case, Reuben had already invested his entire fortune in building the Midas casino. Now he knows to never partner with Willy Bank. While a revenge attempt is possible now, the initial scam was really more of a rule of life that Bank follows rather than something specifically targeted at Reuben out of spite- Bank scams everyone he's ever worked with, even the people who designed the casino.