A few responses, as this is part of what I do for a living. DOn’t know if it is the same elsewhere, but where I live, our CRA (Like your IRS) determines an acceptable proportion of expenses to be spent on administration. It has an acceptable level, a “flagged” level, and an unacceptable level. I imagine the IRS is the same.
As for events, our system has a component called “value advantage” so that any proportion of a donation at an event that gives some value to the participant is not tax receiptable. OTOH, as someone said above, if there are sponsors who have covered all of the costs of an event, this does not apply.