Whilst I don't see how Water Orton bares any relevance to University here goes. As said above, the acceptance by 99.9% of guards does NOT mean the route is strictly valid. I won't be getting into a discussion about reasonability vs validity again, though.
The fares check rule deems appropriate routeing points for Water Orton to be Nuneaton or Birmingham. A journey may be undertaken via any of these. Tamworth is not a valid routeing point by the fares check rule so any journey must not touch Tamworth.
But wait, there is an easement for this journey:
030199 Customers travelling via Derby and Lichfield Trent Valley may travel via Tamworth. This easement applies in both directions.
So you may actually go via Tamworth too if you so want to!
Then we have this double backing through Water Orton issue to contend with if you go via BHM. Looking through the list of easements throws up
000027 Journeys from Derby or Leicester or via Derby or Leicester to Water Orton may double back from Birmingham if the train does not stop at Water Orton. This easement applies in both directions.
which in itself is reasonable because the Cardiff to Nottingham service doesn't stop at Water Orton, but there is also
030052 Journeys via Tamworth and Birmingham New Street may double back through Water Orton. This easement applies in both directions.
further confirming validity.
Checking maps now, Birmingham to Nottingham is valid via BG (via Leicester and then further via Derby if you so wish) or CE (via Tamworth or Lichfield then Derby).
I won't get into checking Nuneaton as that's unnecessary here.
Therefore Water Orton to Nottingham via Birmingham New Street despite double backing at Water Orton and going via Tamworth when forbidden through the fares check is valid through the routeing guide and its easements.
There are no such easements for University, however, hence the question of its validity via Birmingham.