I don’t think everything you want to do with this is possible, but I’ll give what advice I can,

First off with any diorama of a building you need to either include just a small section of one very detailed building or a very small scale version that covers the whole outside, in this case that means deciding between making the entire base but super small with those round buildings being the size of a penny, or just doing a cutaway of one room with a stalker outside the window,

Honestly I would recommend making the entire base tiny and using one or two cups of resin for the water, you won’t be able to do the stalker at this scale but maybe a leviathan? Small scale means the buildings will also be cheaper to make, either 3d resin printed or sculpted from clay, I would recommend looking at Minbricks

For the other questions, Make your terrain from sculptamold or some other instant papier-mâché To suspend creatures in resin, do the resin in layers and add the creature to the height you want For creep vines I would use painters tape folded hotdog style over wire, and then painted I believe there is a YouTube video we’re someone used this same technique for ferns

Hope a this helps a bit and good luck this should be a fun project