If a Photo library is moved to an external hard drive, can those photos be accessible while at the same. time iPhoto still is on my Mac? Can I erase photos in the Photo library on my Mac while preserving access to them on the external hard drive?

When you are using more than one Photos Library, the photos and videos in the different Photos Libraries are independent of each other. You can delete photos from one library without deleting the copies of the same photos from other libraries.

You can copy a Photos Library to an external drive and use the original on the internal drive as well - just switch between libraries by double clicking the library you want to open.

Any external drive you are using has to be compatible with Photos - prepare the external drive as described here: Move your Photos library to save space on your Mac - Apple Support