It sounds like you're thinking of Feynman's popular book QED (which I'd agree ought to be great for people who want an actual idea of what's going on without ta

It sounds like you're thinking of Feynman's popular book QED (which I'd agree ought to be great for people who want an actual idea of what's going on without taking a college course) -- but in chapter 3 he shows how "we can get the correct answer for the probability of partial reflection by imagining (falsely) that all reflection comes from only the front and back surfaces... what is really going on: partial reflection is the scattering of light by electrons inside the glass." This comes from adding up the coherent effects of that scattering -- he wasn't saying the interference goes away (becomes incoherent). It's not my field, but my impression was that path integrals (sum over histories) did initially figure mainly in discovering the Feynman rules for QED, without getting used much directly by others, but later did find more applications.