debsgr8,
Workunits are removed from Results Status on an ongoing basis. Sometimes they are not even there for a day before they are removed. Results Status would quickly become unusable otherwise.

Correct, results not shown until valid

From the results status page you also posted a picture of.

Can see there time being bought when a task errors or goes into pending verification. With invalid, there's already a quorum, no reason to delay assimilation beyond the regular clearing times.

Unfortunately, once they're uploaded back to the project from your host the only place you can see them (failed tasks) is on the results status page. How long it stays up is both up to the project, and dependent on how quickly the task is completed and validated by other user(s). On some BOINC projects, you can look at your tasks up to a year or so. As soon as your task comes back as an error, another replication should be sent out to another user--which usually has a shorter deadline.

"Pending Validation" means the project is waiting for another replication of the same task to come back, so yours can be compared to it. It can be marked "invalid" or "inconclusive" as well, depending on how the others finish up and compare. There is a maximum number of replications that can be sent out, so a "bad" task doesn't clog up the system indefinitely.