I can reproduce this issue. A Butterfree in a Dream Ball would be classified Legal, but changing that to a Caterpie wouldn't. Appears to be an issue with PKHeX not recognizing pre-evolved lineages.
@Kaphotics
That .sln should be in \PKHeX-master\, not in \PKHeX-master\PKHeX.WinForms\.
If for some reason the .sln is in \PKHeX.WinForms\, move it up one folder and try again.
https://github.com/ReignOfComputer/RoCs-PC/tree/master/17 - Gen V - Events/Pokemon Dream Radar
https://github.com/ReignOfComputer/RoCs-PC/tree/master/17 - Gen V - Events/Dream World (BW)
https://github.com/ReignOfComputer/RoCs-PC/tree/master/17 - Gen V - Events/Dream World (B2W2)
Note that the Therian forms won't migrate to pk6/7 properly until the next PKHeX release
Met
Origin Game: X, Y, OR, AS
Met Location: Anything that looks legal for 6th Gen
Attacks
Click the Relearn Moves box to create new suggestions. Knock Off should not be a Relearn Move.
PKHeX displays the date based on the user's locale, I believe. Simmsy's computer displays it this way. PKHeX wouldn't allow you to have an invalid entry like that in the date field
Will note that Bulbapedia lists it as Level 5: http://bulbapedia.bulbagarden.net/wiki/List_of_Japanese_event_Pokémon_distributions_in_Generation_I#Game_Freak_Mew
And Serebii's news page also shows it as Level 5: http://www.serebii.net/news/2016/05-February-2016.shtml