Which depends on personal interpretation, standards on what you consider to be well written memorable story, and what not. That doesn't mean he downplays and mitigates importance of others, but simply prefers others over them.
Yes they are, which is one of main problems with this show. Instead of writers investing more in popular well accepted characters, doing build up on their stories they are constantly cycling through cast replacing them halfway unfinished with series suffering from lack of continuity, substantial development and compacted plot.But isn't that how all of the characters other than Ash were treated? If you are talking about Misty, then yes. Brock stayed on for years and if they wanted to do something real for him then if they cared then they would have done it already. Would you would have want Misty is Brock's place doing little to nothing on the sidelines instead of atleast having her own special series with other characters? Gary has been shwon a few times more than he should have been. So he has had great focus. But you need to realize that he changed his goal, so he and Ash won't butt heads like they used to on like how Paul and Ash did. But that's not anyone's fault. And Tracey is pretty much where he said he wanted to be. It's not like anyone is forcing him to be there if he doesnt want to be. Max is the only one that needs better treating since he hasn't been given much of anything out of everyone else.
Once certain character or pokemon leaves in this show, aside from obligatory regional cameo afterwards he gets bury never to be seen again, erasing all importance and impact he left on anime. Which is bad move imo since its only contributing intermittent story, unstable cast turning viewers away.
Way i see it this character rotation always seemed to me like best of both worlds.
With such approach new companions would still be included, but for change older ones would remain relevant in some form going through new adventures growing forward(similar how other anime like "Inazuma Eleven" or "Bakugan" does). That way everyone would be pleased with characters not receiving too much exposure,while at same time being provided enough focus to do something significant with them.
This would if anything bring new angle to this show being fresh for change to follow story of already somewhat developed character going on bigger things, and see in what new exciting directions writers can take them.
Speaking of reference to Brock, he isn't credible indication through which you can base how would other characters be treated. It depends on personality, character goal, plots writers build around him ad story in what way he will be taken and handled, Otherwise Cilan and Iris which came after Brock would be treated same like Brock was, but that's anything but true getting more focus and growth.