You have been repeating this a lot: there's no way the warriors are in league with the Reiss, otherwise they wouldn't have gone till there to destroy the walls and losing a comrade (Marcel, eaten by Ymir). Furthermore, the warriors wanted to kidnap Historia because of her lineage, as they wanted mysterious info concerning the walls. What would the point in doing this be, had they already been allied with the monarchy??
Anyway, yours is an interesting issue, one of Aot's mysteries: my interpretation is that Ymir meant she had stolen the titan power 5 years ago (and not "long ago") when the warriors were reaching the walls to destroy the gate, but maybe it happened before (that's why Annie wasn't there in their flashback). Holy shit I didn't think of that, it must be so!
However, I think she was overestimating the warriors to convince historia, but it may be true that they are almighty, as reiner implied here:
http://www.mangastream.to/shingeki-no-kyojin-chapter-38-page-42.html , in their hometown there are most likely other shifters, who stayed there to protect it in case of an attack (maybe by their true enemy: the beast titan). Even without the coordinate, at full strength (and not tired as they were in volume 12) Reiner,bertoldt and annie would have destroyed all the titans in their way, therefore even Eren would have lost pretty badly.
That is, their ultimate power is their fighting skills
