Zarg,
I do hope Mrs Zarg is doing well, all considering. But if I may analyse your story:
1) B717 flight OK: tick
2) Customer service after being declined: All credit and handled EXTREMELY well. Double tick (due to capitalisation)
3) Only bone of contention: Being refused boarding after arriving three minutes late: Cross
To deal with the cross. By your own admission "traffic and non-familiarity with terminal layout" in combination with Mrs Zarg's lack of mobility left you running late. Given that these three things are issues you knew full well about (ps - these days just assume the traffic will get you - ESPECIALLY in Sydney) do you really think it's Jetstar's "fault" you didn't make it on?
Given that the fourth piece of information you had in your hand was a booking which states in NO uncertain terms that if you miss the 30 minute cutoff you don't get on, It really shouldn't have been too hard to leave home 3 minutes earlier.
I understand why you're upset, but ask yourself this: if you'd made the flight would you be saying good things about the Jetstar experience? Judging by the thrust of your post you would be.
Motto: It's easy to blame others but sometimes they don't make the mistakes - you do.