@Out of Trim:
Although that sounds like a plan initially, since the AP will follow FD1 when the LH pilot is PF, keeping the autopilot on will try to follow the FD1 anyway and still go nose down for example. If it switches to another mode like ALT or PTCH it could lead to more confusion ("why is it doing that"). A disconnect does not necessarily do anything to your flightpath. The plane is trimmed, power is set and very little variables change.
In the mean time, there will be at PIT flashing on the PFD and the EICAS should list EFIS COMP MON caution message. These two combined lead to a QRH checklist, which basically lists "compare EFIS and ISI, switch both PFD to output from the reliable data side (IRU2) and proceed with IRU failure procedure". Please note, this is paraphrase and I'm 700/900 rated, not 200. But I can't imagine there being much difference in the basic procedure.
This is also one of the things that surprised me. They should have had an EFIS COMP MON caution message which would have been a clear indicator to take a moment and cross check regardless of whether the PIT was visible or decluttered on the PFD. Cancel caution light, check EICAS, state the message displayed.
Easy in hindsight of course and we can read all the challenges the crew faced. It just seems like this is one that could have been so easily avoided which makes it extra sad and perplexing that it had this outcome.