I didn’t read every single post, so sorry if this is a repeat.
@homerdog I believe you are overthinking this (not to admonish you because I am often guilty of same). For instance, Carleton doesn’t even consider demonstrated interest (CDS item C7; however they do consider interviews). All colleges are well aware of the many reasons students do not apply ED. Your son’s interest in a college should show plainly in his “Why College X” essay. He can also show interest by doing an interview, attending open house events, getting on the mail/email list, and applying EA where available, not to mention visiting if feasible.
There is no way around the fact that the profusion of ED has had a negative impact on non-ED applicants. You’re doing great to look for ways to maximize the chances of a good outcome, but mostly it is an art not a science. For instance, Carleton offers waiver of application fee for students who do a pre-app in August. Even though they say they don’t consider interest, this is a small easy thing to do that can only look good on an application. No downside except being on the ball in August. (Who knows if this will still be the case in 2018, just an example).