While in the real world ...
I don't see inbound services from Skegness being a problem, as those who needed to be onboard from before Sleaford / Grantham would already have, and intermediate stations between Sleaford / Grantham and Nottingham are unlikely to generate huge passenger flows. I also don't think guards would like to get into arguments with passengers from intermediate stations as to whether the train originated from Skegness or not.
It is the outward services from Nottingham which are likely to need measures to help prevent overcrowding, hence the terms make it clear that Skegness trains are not valid with Red Dot Day tickets, and passengers would know which ones go to Skegness.
Norwich services are quite clearly not intended to be barred. How else would one get from Peterborough to Nottingham? Via Lincoln? Good luck with that.
Poorly worded most definitely, but I can't see major issues. Maybe next time they can revert back to not running this promotion during school holidays, just like last time, so there would be no need for exclusion clauses.