PLUGIN for external Video editing EDIUS Pro, Premiere...
i tried Proshow Director and i hope you develope a module to interface your software with an external Video editing software such as EDIUS Pro, Adobe Premiere and so on.
I did not have any problem using EDIUS together with Proshow whenever I needed them to work together. (Now I can do the same with EDIUS and Photopia)
It depends on how your show will be constructed.
Many photos and a little video - use Photopia ot Proshow and add the videos to it.
Mainly video with a few photos - use EDIUS, add the images and adjust them by using the layouter.
I don't think that having Photopia as a plug-in to EDIUS (or any other NLE) is necessary. Too much time developing it. That time is better spent increasing and improving the features in Photopia.
Here is a short video I made a few years ago for EDIUS users. https://vimeo.com/254979758