Also, the crew would first have to know something wrong is going on either based on activity happening that was not planned previously or unexpected data on flight instruments. But guess what is driving those instruments in a modern crewed space vehicle - also computers and software. That software might be faulty as well or even displaying the same wrong data the automated control software is acting upon.
In such a case the crew might not even notice something is wrong until the craft is on a wrong and potentially even unrecoverable trajectory once ground radar notices something is wrong.
As for the crew taking over thtuster control during a reentry - sorry, if you space capsule is trying to kill you that hard, something is wrong.
At that point in time, the capsule is hurtling through the atmosphere protected only by its from ablative shield. The thrusters are used to shift the center of gravity a bit, to give the capsule some lift, offsetting some of the g forces due to the rapid deceleration. This is called "lifting reentry".
This all needs to be very very precise & based on up to date sensor data, as the whole capsule is not covered by the heat shield and if you change the center of gravity too much, you might expose unprotected parts of it to the hot plasma.
This is not really a good environment for a crew member to take over - not only are you under couple g's of deceleration but any mistake will kill you all. But hey, no pressure!
BTW, the Soyuz capsule has a backup mode available in case it's reentry control thrusters fail, where the capsule just follows an unguided ballistic reentry. This is much harder on the crew (due to no lift compensating for some of the deceleration), but survivable & has been used a couple times during various emergencies.
