Coming up through cracks in my breezeway concrete and along the edges of my driveway are self-seeded Lobelia syphillitica/great blue lobelia, ferns, Persicaria virginiana/Virginia knotweed (variegated), Aquilegia/columbine, Agastache & half a dozen other perennials. I mostly let them do their thing since the bees & other pollinators don't really care where they find nectar sources so neither do I. So yes, your perennials will most likely self-seed. Oh, and by the way, mine do it with zero help from me.

My soil is healthy, slightly acidic sandy loam. I just let them do their thing since they know more than I do about where to grow. The only thing I do is stake them when they need it and bring the stakes in come winter. Oh, and I do provide supplemental water if the weather's dry.

Yes...and no. Generally, seeds need to ripen on the plant so precipitous deadheading can reduce the viability of seeds. Should the seeds have ripened enough to have viability, the soil requirements are, as with most things in life, pretty much down to chance. Some will, some won't, some may, some can't. Don't sweat - there will be volunteers...but whether they are the ones you want, landing where you would like them, can really only be decided with a bit more controlled intervention other than vaguely casting things about...but on the other hand, to get free plants for zero effort - what's not to like?

Re the Echinacea and casting the faded flowers: suspect NM might agree that it's better not to use the flower itself in case it contains coneflower rosette mites.

Sounds like the mite problem is currently being handled, in part, by plant hygiene measures.