actually, ichibe said that he named everything in soul society. that means everything of any worth within soul society, even other people creations, he named. as he was the one that named the zanpakuto. there could be no shinigami before the zanpakuto because the zanpakuto is essential in sending souls to soul society. which means that nimaiyo is ancient as hell. there could be plenty of reasons as to why yama fought juha instead of ichibe.
first of let me say that it's intirely possible that juha is way stronger than he was 1000 years ago. so as to why yama fought him them instead of ichibe, it could be because he wasn't worth his time. furthermore, ichibe is the head of the royal guard, it is only his job to fight juha if he is a threat to the soul king. also, yama didn't have the strongest zanpakuto. what was said is that he had the strongest fire type zanpakuto, just like toshiro has the strongest ice type zanpakuto(which is up for debate at the moment). what ichibe just did would have made all that power yama's zanpakuto had useless as ichibe would have just taken all it's power.
it also was never stated that yama was the strongest shinigami. yama said that no shinigami born within 2000 years has been stronger than him. which we can't really take his word for, because he had no idea how strong aizen was. as for him not facing yama 1v1, he sent his much weaker lacky to face him. all the while juha was just having a chat with aizen. he wasn't sweating yama, he even downplayed his abilities by not naming him a war potential. ichibei is stronger to yama in my opinion. i actually think nimaaiyo could be stronger as well, he just got blindsided by the quincy elite after they got boosted by ashwalen. but before then, he one shotted and speed blitzed all of them no problem. without using any sort of ability other than swinging his sword.