I understand the points you are making and agree with you somewhat. But my experience getting in to BYU and staying there may explain why it's such a low percentage of native Americans (no idea about Pacific Islanders I can only speak for my experience.

I graduated from high school in Canada top ten in my class (of only like 140 kids so no big deal). Got a 30 on ACT and a full ride academic for byu on the native scholarship. I had several lds friends who were also native and none of them had anywhere near my grades and only one applied to byu. She was also accepted with a full ride academic scholarship. For grades and act well below average.

While at BYU I joined several multicultural clubs, I was a soar counsellor and also worked in the psychology Dept working with high school students trying to get their ACT scores up from tribes in southern Utah and Arizona. It was crazy how many lds native kids I met through that program and SOAR who just honestly didn't have any grades to get into a community college let alone BYU. It's not that we didn't make options available for natives it's just that their parents, counsellors and teachers in high school couldn't motivate them to get grades that were even the minimal standard that byu would enroll. And even mentoring them to want to go to college was a difficult chore.

Basically the solution I found was not lowering standards to get more diversity in to ByU but making a church system where mentoring high school students academically before they ever even apply to prepare them for college would be a much better approach. This will bring more students of race to ByU and provide a situation that they won't fail.