Is the system version below your post still correct? OS X 10.10 ? Or have you upgraded the system to a newer version? Photos 5 on on Catalina has been known to crash when trying to import too many photos at once. I have not seen a report about this problem for earlier versions of Photos.
It is easier to keep working with two libraries than trying to merge them, while you are still running macOS 10.10 Yosemite. You could convert the iPhoto Library to a Photos Library by opening it in Photos and simply switch between the two libraries by double-clicking the library you want to work with. exporting a reimporting is a lossy procedure with many draw-backs:
- You are losing the albums and folders
- You are losing the connection between the edited versions and the originals and have to export the originals and edited versions separately.
Converting the iPhoto Library to and switching between the two Photos libraries will preserve the work you have done in iPhoto better than transferring the photos to Photos, and not the library.
Once you have converted the library to a Photos Library, you could use the full, paid version of PowerPhotos to merge the two libraries. But even PowerPhotos has limitations, unless you can upgrade to macOS 10.15.3 Catalina and do the merging on Catalina. The only way to get a lossless merge of two Photos Libraries is uploading the two libraries to iCloud Photos Library. The libraries will be merged in iCloud.
See: Notes on Merging Photos Libraries, 2019 V… - Apple Community
also: https://www.fatcatsoftware.com/powerphotos/Docs/copying_limitations.html
