If probate hasn’t been granted, then the owner of the unit is the estate (and the estate may be the owner after probate is granted). Presuming the appropriate documentation is supplied, I don’t see what the problem is with the estate nominating someone to the EC, but it would have to be the duly appointed representative of the estate. That is a matter for the strata manager, rather than you. You could ask the strata manager if they are satisfied that the nomination is in accordance with legal requirements.
To be honest I don’t think it is any of your business whether or not the unit can be leased. That is a matter for the executrix.