Yes whenever you resample an image via interpolate you loose image quality you either through away image detail you have for an image when you downsize or your making up details you do not have when you upsize in each case you wind up with a new image. You no longer have the original pixels..
However if you save layer files where you use Smart Object Layers when you resize a smart object layer you always resize from the pixels original rendered for the layer's object. So with a smart object layer if you scale it down in size then scale it back up ins size you will regain what you through away scaling down for you not scaling up the scaled down images your scaling from the pixels rendered for the smart layer's object. The document Canvas size does not change. The image will either be smaller than canvas size or be clipped by the document canvas size. Layers can be any size. You can also resize the document canvas. The layers object does not change when you change the canvas size nor does the layers associated transform.
Did this start as a RAW file? We do so much of our essential editing in the RAW file conversion, that I consider the RAW file with its Sidecar file, the master document. If extensive work is done in Photoshop it gets saved to PSD and never downsized. 5000 pixels on the long side, is still a pretty big file though, and would make a very big print.
Yes, this started off as a raw file but saved as a TIFF. So I do still have a raw file in the original size, but the edits are done in TIFF. I guess lesson learned is always save as another file when resizing images.
Yes, absolutely, but in this particular case I wouldn't be too worried. As Trevor said, 5000 pixels is still a big file, big enough for almost any practical purpose.