“But at some schools the amount and/or nature of the demonstrated interest directly impacts the type/thoroughness of review the application gets.”
Never heard that not opening emails can stop an applicant at first cut. Yes, it can be a factor in the holistic review, if you don’t visit and live close enough for a day trip or one overnight.
But many families don’t visit colleges until an admit, many can’t afford to run around doing all that travelling. And distance travelled?? And some schools simply do not track, regardless of other optimization programs they may use.
And the decisive factors ARE in the app. Visit multiple times, get on the mailing list, send emails to adcoms, showup at a college fair, etc, etc, will not overcome a weak app. Or a strong one where the Why Us crumbles.
Maybe OP needs to tell us the college targets or at least the tier.