In the context of water baptism it is clearly immersion just by definition of the word in Greek. When used in context of washing hands it seems it can be used either way, niptō or baptizō. If you could find a passage when water baptism is characterized the same as washing it would make for a more compelling argument. The only passages I have found that used G907 or G909 like that is the one you mentioned (Mark 7:4) Mark 7:8, Luk 11:38, Hebrews 6:2 and Hebrews 9:10. None of which describe baptism as washing or dipping or anything outside of the designation of the word itself. I guess another way to look at it is like this, you can wash something by immersing it; but you cannot baptize without immersing it.
On ritual hand washing and Jewish traditions, I can't speak to that. I just draw conclusions from the Greek and Hebrew as it is written.
That is a great site by the way, glad I could share that with you.