It's entirely possible the plane wasn't tracked the entire time, but the crash was heard by a US submarine operating in the area. Loud noises would have made it clear that survivors weren't likely. I can see why they decided not to reveal sensitive information in those circumstances.

The big-ticket item is probably communications They need one of the DSN antennas: huge dishes that there are never enough of. Ending the Curiosity mission makes room for a new mission without having to build a new 34-meter dish antenna.

What's missing is a filter in the receiver circuits.
You've got a transmitter and a receiver connected to the same antenna. When you're using the (powerful) transmitter, you need to make sure its signals don't end up in the (very sensitive) receiver and fry it.
This filter has to provide something like 150 dB of isolation.

The Russians would have had to launch 3 or 4 (if an unmanned Soyuz launch is impossible) Soyuz within 30 days. This assumes they had 3-4 Soyuz rockets lying around at the launch site, finished and ready-to-launch.
Using a Proton wouldn't be feasible; it's never been used to launch Soyuz capsules so they'd have to manufacture a payload adapter.
The Soyuz could only be launched unmanned if its software were able to do automated rendezvous and stationkeeping, AND if its airlock were able to be opened from the outside.
The same constraints apply that TFA talked about: preparing 1-3 missions at a hugely compressed time schedule.
You also have the additional problem of having 4-5 spaceships flying in close formation, each in a slightly different orbit. That's a bad enough problem with 2 ships, but with 4-5 you need all of them to be manned to avoid collisions and blasting EVA crew with rocket exhaust.

Almost, grasshopper. There would have been an earthshattering kaboom.

It's an ESA project, for political reasons this will use an ESA launcher. They added a Soyuz launchpad to the Kourou spaceport a few years ago to expand their launcher portfolio on the low end.

I enjoyed the Cold War thrillers (both military and espionage) that were popular in the '80s and '90s. Military fiction these days seems to be mostly focused on terrorism, which makes for boring adversaries (rabid dogs that need to be put down versus an intelligent, wily, and rational enemy).