Heh, wait, let me guess.
The Indus and Yangtze are scripted to stop once the two diemos corvettes jump out, so when the leander goes all fubar they just keep going and eventually get close enough to the carthage to trigger jackknife, at which point all hell breaks loose?
Yes. Exactly. And there's a reason the Leander is going all FUBAR. What you've done is triggered a 'bug' - really an ugly hack - that's been in the engine since retail.
When you give a disarm or disable command to a ship, the engine tells all ships that are not the ships you issued the order to
to stop firing at your target.
So when you give the disarm/disable command, you're ALSO telling the Indus and Yangtze to stop firing at the Leander...so it never goes low enough hull to warp out.
We really need a fix for this. In the meantime I'll disable all disarm/disable commands for Gamma. You can just avoid using them.