That isn't known because not all of the September 19th correspondence is timestamped and Grant and on that issue, recalled later, but corroborated by Ord, there isn't a good time. Hence, the message to Halle's may have been sent before, or afterwards.
The other interpretation is simply that Grant and Ord had come to the agreement that Rosecrans wouldn't make it on the 19th, however, even if that wasn't the case, the important point is that Risecrans was operating in real time as to the Confedwrate pickets being driven in. It was as a consequence that it was known that the Confederates would know that they were coming. Furthermore, as discussed previously, it remains unclear how much Grant was comprehending how Rosecrans was seeing the situation that way. What is known is that Rosecrans conception of the plan was to strike the Army of the West in order to deprive them of the ability to pull back to the east via Bear Creek.
Yet, certain users have not even acknowledged that the record clearly demonstrates that the Army of the Mississippi was pressed forward in order to strike the Army of the West on the 19th, which, of course, it was.
As for the point regarding the Army of the West escaping, such is precisely the point, hence why the operation was pressed forward.
Lastly, the April correspondence would not seem to be relevant if being used to describe Bear Creek generally. Grant explicitly stated on the 16th his view that it was crossable.