1. To some extent, yes, but it is really an ongoing process of cat and mouse chase; various weapon systems were always continuously updated through history, as the enemy would gain more and more intelligence on the older variants.
2. If we assume that there are several "right" systems available for Australia, then yes.
3. Yes, it is similar in the sense it is trying to do the same thing, only on a smaller and cheaper scale. It would be logical to assume aegis system is overall better.
4. Yes. Radars and combat management system may track hundreds of targets at any one time, but if enough missiles approach the ship (near) simultaneously, one gets to a bottleneck due to the illuminators. Even withi all the time-sharing, there is still a finite number of missiles one illuminator can guide. Actual number we can't know, could be anything from several to a dozan missiles per illuminator, depending on various factors.
It has to be said that coordinating a strike where dozens of missiles will approach the enemy ship simultaneously is also not an easy task. Furthermore, defense systems like PAAMS and future variant of aegis (with sm-6 missiles) don't have that bottleneck, as their missiles have active terminal guidance. Their bottleneck is either number of separate datalink channels for midcourse guidance or perhaps (in my opinion less likely) firing rate at which missiles can be launched from VLS.
