Hey Avengeil! Hope to see you stick with the mapping of this encounter, sounds fun!
My two cents, and by no means am i a professional. So if i was to guess, i'd say you are using gimp, if so i may be able to help a bit. If it's not gimp, ignore everything i say afterwards :-)
1. I also got very annoyed at my canvas expanding and it took me a while to find out what it was; Drop shadows. often times when i'd use the drop shadows tool it would expand my canvas to fit the shadows onto the canvas. How to fix it? Go to image, set canvas size, and however big you initially set it for, type that in, then move the image around in the preview box until it is lined up again. most times it'll be top left, since drop shadows is set for dropping to the right and down. Sometimes it will increase all sides so, there is a button for "centering" that will pretty much crop the edges.
As a last resort, delete your grid layer, go back to "image", "set canvas size", and just retype in the initial size (in increments of how big your grid squares are) an viola! done.
Oh and when you render the grid, make sure the off-sets are set to 0, or it will not align correctly.
2. Terrain transition. That is what i am working at as well. My best advice, overlap your two types of terrain a bit, pull out your eraser tool, decrease the opacity of it, and go down your transition line a bit. Then go back to the eraser tool, adjust the opacity down even more, and slide it down your transition line again. The end result is a more gradual transition, keep repeating until how you like it. i generally do 100% opacity, 75% opacity, 50% opacity, 25% opacity for a nice gradual fade. Doesn't work in every situation but it may help for this one. Works well with water/sand transition.
If gradual transition doesnt work try this: ,Which ever terrain is slightly taller, the sand vs silt, toss a third layer in between those and shade a bit at the transition line. just a tad, but it will give a bit more depth
3. Shadowing - create a new layer between your ground and your objects, label it "shadows". select your spray paint tool, select a nice tooltip i prefer the faded circle tip, select the black color, and then spray paint your shadows. the rocks, overturned cart, spilled merchant goods, should all have a bit more shadow on them imo.
4. Maybe throw in a rock,refuse, or bone poking out of the silt sea as well?
5. lastly, personal preference of course, but i'd suggest going gridless. With a few tabletop systems out there its gets tricky trying to line things up in each of those programs. Since most of them have a built in grid, let the users add a grid later on. However if you want to keep the grid, by all means, i'd just suggest toning the color down. If you have the grid on it's own layer, just bring the opacity of that layer down a bit. or pick a more neutral color.
anyways i hope this helps and if my pointers don't make sense, give me a shout and i'll try to explain better :-D