I think there is precident in Core for a domain granting access to a spell at a lower level than it is otherwise availible, but I might be mistaken.
Running: Ruceeglaelsktinag ICOOCPrior Map
Power Rangers:"BIG TWIST!!" (working title) ICOOCActive Map.
For everything else (including my extensive homebrew) see: Extended Signature My Homebrew is meant to be used, but, if you do, PLEASE tell me how it goes.