The Rema is really very clear - there are six items on the seder plate plus the matzot. I've never seen anyone who tried to claim otherwise, but if you're seeing other descriptions in modern haggadot, my feeling is that the basic factor at play here is modern revisionism. With regard to leaving out the salt water, most of the other minhagim (Gra, Ari zl, etc) don't have salt water on the seder plate. Since most people do not follow the Rema's order for the seder plate, it's simply unusual. The only people I know of who have tried to keep the Rema's order are the Yekkes (German Jews). Since most people nowadays don't have the salt water on the seder plate, the authors of contemporary haggadot feel comfortable leaving it off "because it's not done" even if they're ignoring what the Rema actually said. Their goal is to give options that work for today's generation. As minhagim change, they print what they expect their audience to want.
As for the matzot - it's a similar story. There isn't room on the modern seder plate for today's modern sized matzot (I have no idea what size matzot the Rema had - they may have been a different size back then). Therefore, people leave it off. Even the German Jews who tried to keep the Rema's arrangement started making multi-layer seder plates in order to keep the matzot on the seder plate technically, but not have them interfere with the other items. Since it's just not done anymore, many authors feel comfortable providing something which is more usable (in their mind) even if it's not technically accurate.