The thing I don't like about this is it seems to kill aspects of Usagi's character to much. Also it isn't really like the Bhepin treatment. Shinji wasn't trained to fight giant robots, he discovered a fun game which involves strategy and space age combat fantasy and helped him grow in personal philosophy. This may have enabled him to be a better pilot to the Eva in rather direct ways, but it is key that he wasn't actually trained to do this, a large part of the awesomeness derives from it being something original and spontaneous and ridiculously cool.
A closer approach to the Bhepin treatment might be to have Usagi discover and play Dungeons & Dragons. They _DO_ play it in Japan, the creator of Slayers was a D&D player who wrote for the Japanese edition of Dragon magazine. Introduced to fantasy to this degree, and having her have played a spell caster with some strategic skill and have some sort of way to translate being a magical girl to herself mentally would produce similar results. In fact, the Sailor Moon random power ups and new spells discovered in crisis closely resembles leveling up by experience. It also allows Usagi to retain more aspects of her original character and not have the limits being 'trained' brings, such as a specific fighting style. If Usagi were to have, for instance, been the Dungeon Master or a party leader, she might have also developed leadership skills and an ability to plan ahead, while still retaining a sort of new-ness to the whole magical girl thing, as well as bringing a new perspective to it.
What made Shinji and WH40K special was that Shinji wasn't expecting or trained to pilot a giant robot, the leadership skills he developed were incidental and related more to morals and luck than any specific training, there is nobody in the background who turned him into a angel killing master.
Giving Usagi training to become a devil hunter before-hand sets a weird precedent, even if her different mystical skills sets her apart from Yohko, she still has the fighting style beaten into her, which kills off the much more awesome prospect of her being a natural at clumsily dodging attacks and having incredible physical attributes and reflexes and great determination, not to mention overwhelming power. Being taught to keep a cool head in a fight might even go so far as to hugely limit her abilities given how they develop most in emotional situations. She also might get weird expectations for how her powers work or what their limits are based on the powers used by Yohko which will limit her. The reason Usagi's crazy new spells work is they just come to her as she needs them if she gets emotional and desires something like them enough and she has an open mind and no idea how her power works. This is similar to how Shinji has no idea of the limits of the AT field in Bhepin's fic, or at least those that are perceived by others, so he can pull off ridiculous yet cool things with it with his imagination, guts, and sheer willpower. I believe Sailor Moon's magic behaves in a not too different way, she develops new powers based on need and circumstance and emotions, she can repeat those powers at will, but the basis of it all is essentially practiced reality warping to give herself repeatable powers one the need builds their definition, but with them locked into something of a theme that is spiritually aligned with the powers.
Training Sailor Moon in Yohko's style is more likely to stunt her growth and make her _less_ awesome, rather than more, because of the way their powers differ. Sailor Moon's abilities are about overwhelming power and need. Yohko's are about training, precision, and control.
Still, you are on the right track with giving her a _desire_ to see fighting the Youma as a serious duty. Notably, in Shinji and WH40K, Shinji refuses at first to pilot just like he would in cannon, but is more serious about choosing to pilot when he realizes Rei would have to instead, and afterward he learns to see it as his _duty_ to protect people from the Angels.
A better approach might be to have her visit a shrine in her youth where she is recruited as a miko although she doesn't get trained to develop mystical miko powers like Rei does, she'll have been told legends of demon hunters and have some vague idea that miko and priests are supposed to fight demons, and perhaps get the weird idea that her powers are drawn from some sort of moon goddess, only later to begin to realize more accurately what the powers are and begin to realize that she herself is some sort of goddess, but by then she'll be determined to protect humanity from the forces of darkness. You could even have her know and be friends with Yohko, but not actually trained to fight, just familiar with a different set of ideals, and trust into a situation where she has the power and ability to help.
A good way to spin it would be to have Usagi be told that Yohko's power comes from her bloodline, so she can't learn it, and she shouldn't endanger herself fighting demons, but that doesn't mean she can't help, and Madoka could convince her to work at a shrine, and that doing so helps appease spirits and reduce the amount of them that go rouge and turn into monsters. All Usagi would really get out of it is some skills at cleaning and greeting visitors to the shrine and mystical lore knowledge of Japan, and maybe some religious rights and dances, and maybe her friendliness and good nature might earn her some friends in the spiritual community and religious folk in Japan and some real mystical expert contacts, but she wouldn't actually develop powers or get combat training until she meets Luna and becomes Sailor Moon.
Training _after_ she becomes Sailor Moon would still be feasible, as the pace of enemy encounters and the initial power developments would keep the training from getting far enough soon enough to limit her, especially if she has he perception that she is 'wishing' or 'praying' for her powers in the form of some sort of miracles and envisions her magic items as holy artifacts.
"Health, Learning & Virtue will ensure your happiness; they will give you a quiet conscience, private esteem & public honor." Thomas Jefferson (1743 - 1826)