MC Escher made a series of etchings using space filling shapes - a form of tessellation, although not uniform tessellation.

Sometimes.

By definition, a semi-regular tessellation must include more than one type of regular polygon. Some uniform tessellations use more than one type of regular polygon, but many uniform tessellations use only a single regular polygon. Therefore the statement is only sometimes true.

Yes it is a tessellation.

Non-visible tessellation or non-existent tessellation, perhaps.

How to Make a Tessellation?