Greetings from North Idaho. Long time reader who decided I should start sharing here. I have been starting milkweed seedlings mid winter for the past two years. This year I have just potted up 250 plants in the last week. Haven't tried all of the species you mention, but have had great success with showy mw (A. speciosa) and swamp mw (A. incarnata), both natives here in Idaho, and a then few Eastern Purples (A. purpurascens) for my garden. Showy is much like common, so would think that propagation would be simmilar. I use a damp paper towel now, as I got tired of having to sort through the sand to find sprouting seeds. They always seem to get burried where I can't see them at a glance. I have found that I get 95% germination of showy seeds with only 2 weeks of stratification and about 60% with Swamp. That bumps up to 85% after 3 weeks for swamp, so really, not as long as the literature says. I place the seeds between two layers of damp paper towel that is squeezed out, place them inside a ziplock laying flat on a fridge rack for 2 or 3 weeks. Somewhere in there I put them outside for a few hours to freeze. As soon as the paper towels freeze stiff, I put them back in the fridge to thaw slowly. If you place them in your freezer in stead, don't let them stay too long. Your freezer is much colder, so 20 to 30 mins. is enough. I found that i killed many of the seeds by leaving them overnight in my freezer. Just a quick freeze is all i do, and even that may not be needed, but why change it if it works.
After 2 or 3 wks I then pull the bags out and set them at room temp (top of fridge, counter top, wherever it's about 70f. Begin checking at about 10 days. I pull the seeds that have a root started about (1/4 inch), and plant them individually in 5"deep plug trays. You could use larger pots if you want. The seedling roots grow [url]very fast[/url], straight down, so bigger is better (they will be growing out the bottoms of my 5" plugs in 2 wks). Keep checking every two days, and pot up any that have sprouted. If you let them go too long, the roots grow through the paper towel, and they are hard to get out without dammage, then you get the rot. I get basically no rotting by checking often. Some species have a longer germination spread than others (mostly 10 to 20 days), but even the seeds that are unsprouted after 20 days will eventually germinate if they are potted up and kept moist. I simply use tweezers to poke a 1/3 to half " deep hole in the potting soil and slip the exposed root straight down while leaving the seed just below the surface of soil. Firm it in from the sides, not the top. Put a few more seeds than you will use in the bags and you will have plenty of sprouts to pot up 4 to 6 weeks later. I did find the Eastern Purples had closer to 50% Germ with 4 weeks strat., and they sprouted over a much longer time, so species are definetely different, but I really prefer to just check the paper towels for germinating seeds often, as opposed to the sand. When started this early, they plant out very well in May, and are well established before the summer dry period. I am naturalizing these, so no summer watering. I do the same with A. curasavica, but skip the stratification. They sprout in 8 to 16 days for the majority.