Unfortunately, the someone on twitter seems to be closer to the mark than the party at the moment.
It's really not that difficult to maintain a membership list. It takes time and administration. It seems like its been badly neglected. Any card failures or direct debit failures are known almost instantly. I know this as I've done it before and you then try to collect payment again. It shouldn't be difficult to update the membership list thereafter. In fact, you should be able to integrate it.
Those eligible to vote should be defined somewhere. Then there is no scope for argument. My understanding is that it isn't clearly defined. However, someone whose membership lapsed 3 years ago shouldn't be voting. If its a tight result there may well be a challenge to the result.
The party is imploding but change had to come. Its one of these one step backwards to take two steps forward moments.
Assuming the new leader grasps the opportunity of a reset. This is a chance to sort the party and government out. Get the party engaging with the membership and improve transparency and decision making. Get the SG delivering competent government for the people with improved outcomes. It sounds simple but it's a bit ask, admittedly. Only 2 of the candidates have any chance of achieving that. I'm not counting Ash.