I don’t think the Harvest Festa ED would be fitting for Rewrite when the serious parts come along… But for sure Common (and Harvest Festa if they decide to do another season with OVA’S telling Harvest Festa events)
I’d rather have those other songs (Itsuwaranai Kimi E, Koibumi, Canoe, Watari No Uta, etc) play in their rightful place. Itsuwaranai Kimi E for Akane, Koibumi for Shizuru, Watari no Uta for Moon, and Canoe for Terra. Those songs were made for those certain routes. Wouldn’t feel right playing then elsewhere.
And I hope they only play those songs at the ending of the routes, for the final episode of their route (if their routes are adapted). Those songs are songs that you should only hear once, to express how important the moment is, and how important the song is for the certain route. Hopefully 8-Bit doesn’t “milk” the songs out and play them after every sad moment to the point where it can literally be the main ED for the season AHEM J.C. W/ Song For Friends in Refrain**
As for the insert songs, I’ve said this before but yah, I hope they use the insert songs (branch album) as Ending songs for each heroine’s route (if they do adapt all the heroines routes). We already have ED’s for Shizuru’s and Akane’s routes, but it would be nice to have one for the other heroines too.