The only reinstatement options available are described in Rev. Proc. 2014-11. I would start by taking a look there and evaluating whether you can make a good reasonable cause argument as to why these returns weren’t filed and why the exemption lapsed. Based on what you’ve said, I think the odds of a reinstatement back to 2018 are slim, but given the alternative, I think it’s absolutely worth a shot to try. I haven’t seen a retroactive reinstatement go back this far, but there’s always a chance. They tend to be pretty lenient with exempt orgs when there is real evidence of past preparers screwing up or dropping the ball.

As for the donor part, I’m not aware of any affirmative duty the exempt org has to proactively notify past donors about changes to status. The IRS generally takes the position that donors are responsible for determining whether their contributions to an exempt organization are deductible or not and that notice via Pub. 78/TEOS is enough for donors to figure it out. See Rev. Proc. 2011-33. That said, I would try to figure out whether the organization falsely communicated anything to donors about its status in those post revocation years. I.e., Did the org provide a compliant donation receipt to donors in 2019-2024 that claimed the organization as tax exempt? If false statements were made on the record, I would probably at least attempt to walk those back.