Digital BW, The Print

Am I destroying image quality by my workflow?

1)  I shoot JPEG and convert to 16-bit using the "mode" in CS to do editing. I convert back 
to 8-bit to save and before printing. Am I actually achieving anything? Am I affecting my 
image negatively?

2) If I change the image size up and down, does that alter the digital file or affect image 
quality? I usually save the image in its original file size/resolution, but sometimes change 
image size several times on the file I'm using to print.

Cheers,
George Le Masurier