True, true... but there are plenty of street and place names whose current spellings diverge from their etymologies.

The only other source for the possessive form is TIGER (notoriously bad in this regard.) Meanwhile, the street signs say MARKS, the imported addresses say Marks, the PAD file says MARKS, GOAT says MARKS, ZOLA says MARKS, DOB says MARKS, and the bus and bike maps say MARKS. I'm pretty sure that's the correct form.

I would have put Saint Mark's Place in as alt_name, but these ways are already tagged alt_name=East 8th Street... maybe Saint Mark's Place would actually be a more useful value for alt_name though.