What is the total number of isomers, considering both structural and stereoisomers of cyclic ethers with the molecular formula C4H8O? According to me, it should be 11 and this is how I have worked out as shown in the attached diagram. 

However, I am not sure whether all these 11 isomers can be considered as those belonging to cyclic ether.

I had erroneously uploaded incorrect diagram earlier. Now I have uploaded the correct diagram. My specific query is whether the 11th Isomer shown in this diagram, wherein Oxygen is not part of the ring, can be considered as Cyclic Ether.
I would not include the last structure, even if it was clever of you to think about it.

I think that the term "cyclic ether" restricts the scope to "cyclic compounds which have an ether moiety inside the ring". Your last compound would be considered as a linear ether with an alkyl and a cycloalkyl substituents.

Oh and your 6th structure does not have 3 isomers but just 2 because the (R,R) an the (S,S) isomers are actually equivalent by rotation of the molecule. This compound is said to be "meso".