It sounds like you have a good plan of attack, I would mix all but the sea oats and spread where you want them.
I have only seen sea oats growing in sandy dunes so I don't know how they will do in a real soil situation, but they are definitely worth trying, they are such a lovely plant.
My pastures now are a mix of around 18 different plant types and species and they all play nice together and the animals love the mix of choices.
I got the sea oats from Johnny's seeds "northern sea oats Og." Chasmanthium lafifolium. When I bought them I didn't know that most Sea oats grow in sandy soil. I guess the name should have given it away. So you think direct sowing into sod/grass will work? I wasn't sure if I should make trenches or break the ground up.
It would make my life so much easier if I could just, heavily, broadcast it. I'm a huge fan of clover. Early in the spring the white clover is one of the few food sources for the bumblebees.
Sturdy, glittering perennial grass.
Strong, stiff stems produce copper-colored, airy seed heads that add sparkle and graceful, arching lines to arrangements. A resilient accent plant in perennial garden beds. Plants are bright green early in the season (June through July); in August-September, the spikelets transform to beautiful copper color and the leaves turn to a contrasting lime green. Seed heads are 4-6" long, each producing multiple spikelets 3/4-1" long. Produces inflorescence (seed heads) in the first year from an early-spring or late-winter sowing, but plants will be short, about 12", and produce small clumps of flowering stems. In the second year of growth, plants are much more robust, producing more stems and seed heads, and reaching their full height potential of 24-36". Pairs well with almost any color in arrangements, but especially creamy yellow, salmon, blush, and white. Holds well both fresh and dried. Native to the Eastern U.S. and Northern Mexico. Also known as Indian wood oats. Perennial in Zones 5-9. Organically grown.
I would think that given the natural location of the sea oat family, you would need to loosen up the soil and make sure it drained well for them to grow at their best.
I think they will grow in soil, but they will need a soil that can drain water away from the plant crown at least fairly quickly.
If your soil is a sandy type, then you could probably just broadcast with no prep work at all.