I’m not finding direct yes/no answers for you about cutting down wildflowers and mulching.
This article points out that if you are seeding or allowing your plants to seed, mulch can inhibit growth. On the pro-mulch side though, it also inhibits weed seeds, which will arrive faster than many gardeners are prepared for. The type of mulch matters, too. For example, bark is discouraged because it can mat and form a water-barrier on the soil. Arborist wood chips are mixed materials which don’t mat the same. Composted mulch is maybe too rich for some wildflowers and is great for weed seeds.
Cutting down your plants is generally done to tidy the area, discourage disease carrying through the winter, and it allows you to see those invading problems sprouting. Exactly when you cut it can vary. Are you wanting seeds to drop or keeping the cover in place for wildlife? Do you have disease issues, particularly fungal leaf problems?
A caution I’ll mention is that you may find reseeding annually is helpful. How many viable seeds are produced, and the amount birds and mice eat, can vary year to year. 
I found a meadowscaping handbook, here, from West Multnomah Soil & Water Conservation District. You may find it helpful. Please ask follow up questions as needed. You’ve got a great start on the project.

Thanks so much for such a swift and thorough reply. Thanks too for the meadowscaping handbook. I have just a single follow-up question.

I get the point about removing some of the plant matter come fall to prevent disease, though I don't see a reference to that in the meadowscaping guide or the article. In fact, Ms. Pokorny writes, "Make sure you leave the dead plants long enough so they drop all their seed." And, as I haven't noted any disease problems, perhaps I shall err on the side of mostly letting the plants be so they can drop their seeds. I'll also add more of the same mix (from Silver Falls Seed) in November.  

I will probably cut back some of the taller plants. Is there at date at which I can be sure all the seed-dropping has taken place?

As for mulching, I think I'll see how it does without it, or I'll only put down woodchips on the perimeter to slow weeds there. (I don't have money for fancy mulch, just chipped trees.) Happily, the beds are at a school, so I have a lot of eager students, ready to weed!

Thanks again for your help!

Patrick

Regarding cutting down foliage, I’m sure you’ll find the right timing after a year or two. I have some plants which dry, break off naturally and are easy to tidy. Others turn to black mush after October, and smother neighbors with slime. 
There isn’t a firm date for seed dropping as it depends on the plant. You can watch for the seed heads to empty themselves. It’s a science project in a way, as the method of dispersal varies with plants. I don’t see exactly what you’ve got growing, but I’m thinking of seeds on a fluff like dandelions, or Geranium which launches the seed, or Lupine by “curling” (Info).
Enjoy the process.