>your panniers look very much like backpacks that you cut the straps off<

Yes that they are... sown together so like a saddle bags

>Are you using some kind of special below the seat rack for the short panniers? <

No they fit on ok without extra rack.

Yeah I have 16000 ks on mine so far.... but now I've made two other recumbents [a long one and a folder] so now the 3 of them have to wait their turn. All fun
Happy riding