Okay, lets clear up a few things on the Mandalorian Warriors. The terms Mandalore, Madalore Warrior, Mandalorian, Mandalorian Warrior, Jodo Kast and Fenn Shysha ARE all EU. The fact that Jango Fett and Boba Fett wear armored battle suits IS canon. The battle suits belonging to a group of evil warriors defeated by the Jedi during the Clone Wars IS semi-canon/semi-EU because it is mentioned in the Empire Strikes Back novel.
Originally, Boba Fett was suppossedly the last Mandalorian Warrior. With his fellow warriors gone, Boba was out of work. Therefore he put his skills to use in bounty hunting. Now it seems that the story was pushed back, with Jango Fett as the last Mandalorian and Boba just carried on the tradtion from his father. It is believed that Jodo Kast is NOT a Mandalorian Warrior. He somehow acquired a Mandalorian battlesuit and tried to "replace" Boba Fett. That eventually cost Jodo his life (Boba killed him). As for Fenn Shysha, I know very little on him/her. Only that he/she wears a Mandalorian battlesuit. There suppossedly is yet another being who wears a Mandalorian battlesuit. This being is refered to as Dala when Boba Fett confronts Jodo Kast. It is almost certain that Fett wasn't referring to Admiral Dalla (notice the spelling difference).
I always thought Jango was the last Mandalorian warrior and Boba just inherited the armor. Of course, I've read little EU and don't know much of that. Seems I've been ruled out by proffessional authors though... so where do Jaster Mereel and Montross fit into all of this? Or were they created just for Bounty Hunter and Marvel? We need a writer to pounce on the possibilities of Jango's history...
Boba Fett is reguarded as a legend (in the SW galaxy) and many stories surround him (and his father). Jaster Mereel was one of many aliases he used. If Jango was the Mandalorian Warrior and Boba learned from Jango, then that technically makes Boba the last Mandalorian Warrior (even if he didn't complete his training). Remember that EU is based off of canon. That just means it isn't "absolute." EU provides wonderful inspiration, but when dealing with canon, EU has virtually no effect. Very rarely will an EU item become semi-canon and even more rarely canon like double-bladed lightsaber (first seen in The Sith War wielded by Exar Kun). It all comes down to was it in the movies or not.