So first, I would not use "eligible" in any of the above descriptions because you're excluding these employees from being classified as eligible employees.
All are permissible classifications but the second two need some clarification - the $75,000 needs a defined time frame (look-back year?) and further defined - is it $75,000 in compensation, and is it gross, plan, commission income or base pay?
Why are you concerned with BRF? You need to satisfy coverage for whatever plans/portions of plans you want this exclusion, and if you can satisfy coverage for that then BRF won't be a problem.
The best method is detailed time tracking, but it is a chore most people hate.
For admin, it doesn't have to be super detailed, just your basic steps (census work, testing, trust accounting, form 5500, etc). Tracking these tasks will give you the metrics you need to split the workload appropriately, where to focus training, if some plans could be shifted to another admin, if you are under charging the client, etc.
Here’s a link to class Prohibited Transaction Exemption 92-5: https://archives.federalregister.gov/issue_slice/1992/2/11/5018-5021.pdf#page=2
Among the conditions, the plan must pay no more than the insurance contract’s cash surrender value (or, if less, the participant’s accrued benefit under the plan).