ICANN has make itself accountable to no one.
I was the first (and only) person ever to be elected by the public to ICANN's Board of Directors.
Because I tried to exercise even the most basic of oversight - in particular to examine the financial ledgers of ICANN - ICANN erased even the concept of public elections.
The question of accountability always involves the question of "accountable to whom".
The standard answer for corporate forms, such as ICANN, is "first the board of directors and then to those who chose who will be members of that board."
ICANN's board has historically acted as a toothless body of worthies who do not control ICANN but, instead, hand all real authority to a President/CEO, who like many CEOs is more concerned with empire building and poo-bahing around than in actually focusing on the quite small job that ICANN is supposed to do. (More on that in a moment.)
ICANN has created a system of selecting board members that is self perpetuating and allows only conformists to be considered. The public, for whose benefit ICANN obtains its legal existence, has essentially no role in that process.
Thus the question of "to whom is ICANN accountable" may be best answered with the single word: "nobody".
As for ICANN's proper role: ICANN's job is to assure that the top tier of the internet domain name system quickly and accurately turns DNS name query packets into DNS response packets without prejudice against any query or person making a query. That's a pretty tiny role. It is certainly a role that can be done with fewer than ICANN's hundreds of employees and world spanning array of offices and an organizational structure that would make a Byzantine emperor blush.
ICANN chose California as its home. Nobody forced ICANN to make that choice. And California, like many other states and countries has very rational and well tested method of holding public-benefit corporations such as ICANN accountable: ICANN can recognize internet users as members. Under the law, if there were such members, those members would have many rights to take ICANN to the wood shed should they feel it was running off the rails.
But ICANN has edged around the edges of the law. California automatically creates a membership structure if there are elections for board seats. I competed for my board seat in what looked and smelled exactly like an election. But in order to evade the law, ICANN called it a "selection".
ICANN has not really improved since then. Rather it is simply become more expert at building walls between it and those affected by its decisions and at ramifying its organization so that it is nearly impossible for anyone except the deepest of insiders to comprehend what ICANN is doing or has done.
By-the-way, in case things this is a tempest in a teapot - ICANN's cumulative footprint on internet users amounts to a tax that is somewhere between $1,000,000,000 to $2,000,000,000 per yer that is levied on a non-consenting community of internet users.