Kickstarter does not allow for pornographic material to be fund raised for, and you can only use funds for the product you are working on. So if Frontwing says the 18+ is a separate version (which makes complete sense), then the funds cannot be used to release the 18+ version. The only way I see this working is if the two have an under the table agreement to allow funds to be used for it, but then if Kickstarter comes knocking, Sekai could be in violation of the ToS by using funds for pornographic material.
Looking at it like that, this may just be the limitations of Kickstarter ruining it for Sekai Project.
Herkz was on reddit discussing the content that is cut from the all ages version, and from what it appears, the content cutting might not be as smooth as we had hoped and more than just h scenes might be missing. If herkz is being accurate about it (someone else posted untranslated screenshots of some scenes), then this might not do well at all.