You are also assuming homeschooling happens in a vacuum and there is no such things as co-ops or prepared materials. Your also making the point a properly motivated and disciplined 18 yo cant teach themselves trig?
Calc is not taught in most high schools so lets throw that one out. I know I could teach it to my kids but when I was in HS I know most kids did not take it.
algebra: I would say close to 100% of people who want to and put in the effort can teach a small hand full of kids algebrea to the Senior of HS level.
geometry: 90-100%
trig: 90-100%
Now lets put these homeschooling people in a community, which they usually are in, and extend that network so they either know someone like myself who can teach those subjects on a college level (at least the one hundred level), or someone who knows the subjects well enough to lend a hand and youre damn near 100% for both.
Your turn: What percentage of kids coming out of schools as seniors have a proper and robust enough understanding of these four fields that they are prepared for college? I tutored many who were not..
The answer to my question is - it doesn't matter. All a parent needs to be able to do is to be smart enough to either teach the subject or be smart enough to get them to someone who can.
And FYI most high schools do teach Calc