I disagree that the Immobiliare is some sort of pointless plot device.

Gilday himself lays out the reasons why Michael would want it: This deal with Immobiliare can make you one of the richest men in the world. Your whole past history, and the history of your family, will be washed away.

So, there's a financial impetus for it (and just because you're a billionaire doesn't mean you don't want more money; quite the opposite).

More importantly. Michael's whole life has been spent in a (hypocritical) pursuit of legitimacy. What could be more legit than a major partnership with the Vatican? The Archbishop shrewdly discerns Michael's longing for legitimacy and uses it against him in the negotiations.

The Immobiliare deal, and the accompanying situations are a natural progression from the first two films.