There were 5 teams given a warning by the RFL re Stadia - they can only bring one team up from the NL, I think bringing two in at once would be pure folly. So by my reckoning, if us and Cas cannot agree on a groundshare, then at least one of us would still be in regardless of work progressing on the stadium.
They might as well tell all the teams with stadiums that are fit for the elite competition not to bother going through the application process - if teams that don't meet that requirement are first in line for the chop (regardless of how well they score on the rest of the application issues)