Also, will EPiStore understand that different variations (attributes set differently) of the same cart item are not the same? If not, adding a size M t-shirt and a size L t-shirt will result in "merging" the two to one row in the cart. Or is there another way to stop the cart from merging the items?

Correct me if I'm wrong, but I disassembled the Dll and found out that:

1. It is possible to set any value of an attribute, but not put new keys in the collection. So I can put free text in an attribute as long as I register the key in the XML (value choices won't matter since the set method doesn't compare to them). 
2. Differing attribute values will cause the items not to merge.

If you are using standard admin site for delivering orders than you will get a "Cannot find name for attribute value" exception when you try to open them. because epistore expects the values to exists in the options list.

If you are using standard admin site for delivering orders than you will get a "Cannot find name for attribute value" exception when you try to open them. because epistore expects the values to exists in the options list.

Ah, too bad, I never did that. Actually the solution I was sketching back then never went into production.