Drag and Drop doesn't work well with Photos--nothing you can do until Apple fixes it.
However, there is a workaround--you can export your picture to the desktop (or anywhere), and then use that to insert into PowerPoint.
The Office suite must be updated to use the Media browser with Photos--nothing you can do until Microsoft fixes it.
You can still use iPhoto until Photos matures. However, the version you have must be updated for Yosemite. See this user tip I made to help you get the current version of iPhoto: Can't Update iPhoto because it is not available in the App Store
The Photos icon on your Dock is an Alias to the Photos application. You can drag any app onto the Dock so that it stays there. If you decide to keep using iPhoto, you can drag that app to your Dock and remove the Photos app by dragging it to the Dock.
Photos and iPhoto use separate Libraries. So, if you decide to use iPhoto, any changes you made in Photos won't appear in iPhoto.