Hallo Regi, with the correction possibility of your objective, you should be able to get very good images, regardless of the scope or mounting thickness (within boundaries of course). With our inverted scopes, we're lucky in that the specimen is always in contact with the coverglass, that really makes good imaging easy-peasy
Anyway, by now I doubt the identification of your specimen. I would have a look at Calcidiscus leptoporus, but really, I do not know enough about the plankton composition on your (holiday?) location.
My first image of Coccolithus pelagicus was shot with a LOMO waterimmersion objective 85/1.0, the last image with the olympus 60/1.4.
Good luck hunting,
René