On a first glance the whole concept of non-multicolumn index with included attributes seems...well, just difficult to understand. But I expect for SP-GiST this must be single key with multiple included attributes, right? I couldn't find a test that checks impossibility of on 2-column SP-GiST, only few asserts about it. Is this checked somewhere else?
Yes, SpGist is by its construction a single-column index, there is no such thing like 2-column SP-GiST yet. In the same way like original SpGist will refuse to add a second key column, this remains after modification as well, with exception of columns attached by INCLUDE directive. They can be (INDEX_MAX_KEYS -1) pieces and they will not be used to create additional index trees (as there is only one), they will be just attached to the key tree leafs tuple.
I also little bit corrected error reporting for the case when user wants to invoke index build with not one column. Thanks!