Compost-only Sheet Mulch

A client would like to me design a small (80m²) food forest (only with trees native to Brazil). The lot today is mostly short grass and weeds and the soil is a hard clay low on organic materials, the typical urban soil around here.

Thing is, she wants things to be planted as soon as possible. My idea is to physically remove the grass (by e.g. cutting the upper layer of soil, any better idea is welcome) and fill the elevated beds (some 10cm) with compost. I would like to avoid uncomposted material and, above all, paper and cardboard, to speed up the process. With only compost I believe it shouldn't take long for worm to mix the material with the topsoil and hence allow planting.

(I think it's better to wait a while until things are mixed because the compost is going to settle quite a bit, and planting trees directly onto compost doesn't seem like a good idea)

What do you think of it?

Instead of removing the grass and roots just chop and drop just before you spread the compost on thickly.
Soil improvement, no matter how intensive you work at it, unfortunately takes about 6 months at least to really take hold.
Mulching over the compost layer will help to hold moisture in and give the worms time to pull the compost into the soil.

I would press for giving you a year to get the soil ready for the beginning of the food forest, that way you can get the succession stages ready and have some of the understory already in place to help support the canopy story.

To break up the clay you will need to be able to incorporate some gypsum, lime and other loosening amendment materials into the compost layer.

Some deep rooting vegetables that you can chop and drop once they have penetrated the clay would be a good first crop too.
The organic material left deep in the soil is only going to make things better faster, other wise you are going to be a long time in getting deeper soil improvement going.

If you have to plant trees quickly, you are going to need to be able to mix compost with the dug out soil so filling the hole also adds goodness to the root area and I would make those holes at least twice as large as the root ball.

You could do a layer of burlap or sisal bags (like coffee beans come in here up in the US) then lay your compost down. That should work wonders to help keep the grass and weeds from coming back through the compost layer. I think burlap breaks down completely in six months or so (possibly faster in your climate), and will add some diversity to the mix that should help loosen things up and add some extra fertility to the soil.

Reading carefully, you said plant rather than transplant. So that indicates you want a seed bed correct? Therefore the type of soil the trees naturally seed themselves into is the goal. If it is clay with heavy leaf litter then you can create that with a compost leaf liter sandwich. If their normal habitat is sand with heavy leaf litter it will be more difficult to get root penetration. Is it her desire to have the plants in a raised bed? Wil it take frequent irrigation to keep the clay hydrated or will there be sufficient rain if the surface is protected from sun and wind?
If the aesthetics will not be a problem for some time or are acceptable to the client you could try this method which is on wet clay soil.