Can an estate trustee be required to pass accounts partway through a lengthy administration, not just at the end?

Yes. An estate trustee doesn't necessarily have to wait until the estate is fully wound up to pass accounts — where administration is expected to take a long time, an interim passing partway through lets the court, and beneficiaries, review and approve the trustee's work up to that point, rather than everything being reviewed only once at the very end.

This can be useful for estates involving ongoing litigation, complex assets that take years to sell or wind down, or long-running trusts created under the will, where beneficiaries might otherwise go a long time without any formal check on how things are being managed. An interim passing that gets approved also gives the trustee some real protection for that period, so they aren't left carrying open-ended exposure on everything done years earlier by the time a final accounting eventually happens.

Either the trustee or an interested beneficiary can generally raise the idea of an interim passing, and a court can also order one where it seems warranted given how long the administration is taking. Because timing and cost matter here, it's worth discussing with an estates lawyer whether an interim passing makes sense for a particular long-running estate rather than assuming one is required.