So there will be rye colonized spawned to bulk in a monotub. My question is, is it better to mix the casing/manure/spawn together or to layer it and only mix the spawn with manure then place casing on top?

Better is hard to say since alot depends on growing conditions, but i have had success in just mixing spawn and manure 50/50 and then adding some more manure on top to cover any seeds that are showing.

No casing needed after its 100% colonised.

I,ve found the simplest way is to layer . Lay down layer of manure vermiculite blend then break up colonized mycilium rye on top followed by layer of more manure . I like to minimize contact with colonized mycilium and work fast . Rubber gloves alcohol etc..

Casing is not mixed with the substrate. Casing is a layer that sits on top of it all, provides a microclimate and limits water loss a bit.

No huge need for a casing layer with cubes, but essential for Copelandia cyanescens and others.

I think what will happen is what felnik said. What is the ratio of vermiculite and manure, 50/50? And also could one place dry vermiculite onto top as a buffer? It would be heart breaking to get a contamination.

The myc will look after itself once fully colonised. If you have a good procedure there is little to worry about. The buffer layer you speak of is called casing. You can do it or you can leave it out, I have not had a problem growing cubes with no casing.

Really it all comes down to what species of mushroom you are trying to fruit.
Certain species of mushrooms like Pan Cyan require a casing layer to fruit, others like Cubes do not. If you could answer that question I could give you advice on how and if you need a casing layer. But generally you don`t put manure in the casing layer, while risk of contamination is LOW their is still a risk. You could make you casing layer a little rich with the addition of some BRF(brown rice flower) though, I`ve had great success with this in the past.