What would an optimal unarmed PKM defense be against a machete attacker (the worst case scenario, since armed defense has more options)?
Think of it as fighting different size opponent....You will never fight them all! I think in one of James Bond movies there was machete fighting...Since its heavy and requires more arm swing to it but it's certainly not a stick as it cuts. However it is definitely not a stabbing tool!
Think about the slash defense and this is what you basically use followed with the rest of the principles. Instead of a seven inch bayonet you've got two and a half times of slashing tool.
Remember however that you cannot afford projecting yourself as it will go for your limbs first, and that you don't have too many chances like in the James Bond movie regardless if your assailant had watched it or not...
Pure Krav Maga provides training with minimum sufficiency and therefore you need to look at the training as pure physics and the connective formulae. You have just the sufficient amount of techniques which are the practice tool for all scenarios to drill you using your judgment to prioritize, predict the possibilities, and counteract an any possible attack.