After carefully replacing the MMIC on the PCB, I tested the IF module by applying a 70 MHz signal again. This time the signal made it through all the way to the output as expected. So I put the case back together and installed the module in the DAR Plus receiver. And, as expected, the thing started working again.
And what do we learn from this? Never assume something can’t be fixed in-house. Any TV station should be able to to this themselves. And if you don’t have the necessary equipment, that’s also fine. Ship the device to me, I’ll fix it for a fraction of what the manufacturer would charge you.
Links and Sources:
 MRC DAR Plus, Vislink: http://www.vislink.com/
 INA-02184, HP: http://www.datasheetcatalog.org/