I can't tell you how late you can plant for your area - much depends on your first hard frosts and how deeply the soil freezes in winter and the amount of precipitation. In my neck of the woods, fall planting is de rigeur - the temperatures are cool and mild, we seldom have hard frosts and winter ground freeze and we always have adequate fall rains. Fall planting allows trees and shrubs to establish root systems over winter, giving them a head start on spring planted items. They can take advantage of the natural irrigation from fall and winter rains and thus become less sensitive to summer heat and drought than newly planted spring material. This is a pretty abbreviated explanation, but if weather permits, fall planting is always recommended as conditions are more favorable to the plants becoming established rapidly.
'Topsoil' is a pretty generic term - it could be anything from scrapings from the nearest new development to an actual soil mix prepared by a landscape supplier. A good quality soil mix should not harden over winter. OTOH, one should not leave soil additions in layers - ie., a layer of native soil, topped by a layer of "topsoil", followed by a layer of mulch. This tend to impede drainage. Better to incorporate your additions into the native soil by tilling, preferrably a couple of weeks before planting so that the newly tilled soil has time to settle. Mulch typically is considered a top dressing, so depending on what is used, you can rake off and save for reuse before tilling or incorporate into the mix when you till and reapply with fresh material. I mulch with a compost product - this is a great additive for any soil improvement project as well as an excellent moisture retentive and fertile mulch.
pam - gardengal