Your topic's Title seems to be one question:

"If you are a W-2 wage earner and have a Sch C Business can you deduct SE Health Ins premiums"

Yes; that's exactly what happens for an S Corp greater than 2% shareholder-employee.

Read about it here: https://www.irs.gov/publications/p535#en_US_2020_publink1000208843

"on Line 10a of the Form 1040 derived from Sch 1 Line 16?"

It carries over as deductible, up to the net income from the related business; the rest would carry over as part of itemized.

The Body of this topic seems to be a different issue entirely:

"it may be more beneficial to claim those premiums as an SE health insurance deduction instead of an itemized deduction."

Medicare coverage is treated the same, as would be marketplace coverage..

Is the taxpayer eligible for health insurance through their employer?  If so, no, they would not qualify (assuming the employer insurance is "subsidized").

Otherwise, yes.

Or, if they are covered by an employer plan, even if they pay their own premium, that isn't "established" by their own name or their own Sched C business, so in that case, it doesn't qualify.

I just replied to a topic where I used the word "Matrix" and this topic is another example of "a matrix of conditions to consider in combination" because there is no One Right Answer.